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IRVING'S 



Catechism of Astronomy. 




Rewritten by 

ANSELM ORTMANN, O. S. B.. St. John's University, 

COLLEGEVILLE. MINN. 



Adapted to the use of Schools in the United States. 



JOHN MURPHY COMPANY. 
publishers: 

BALTIMORE, MD.: NEW YORK! 

200 W. LOMBARD STREET, 70 FIFTH AVENUE. 



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COPY B. 



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Copyright 1905 by 
John Murphy Company. 



PREFACE. 

The object of this Catechism of Astronomy is to present 
the fundamental facts and truths of astronomy in a clear and 
succinct manner. The catechetical method has been adhered to, 
as being best suited for the yet immature minds of children in the 
advanced grades of the primary school, for whom this catechism 
is mainly intended. Possibly, the little volume may also prove 
useful as a convenient reference book to look up the more common 
data and doctrines of astronomical science. 

The changing of night into day by strong artificial illumination, 
especially in the towns and cities, has made the starry firmament a 
strange object to many, and it is safe to say, that, comparatively, 
only a few have more than a vague idea of the diurnal motion of 
the stars. It seems, therefore, advisable to direct the attention 
of the young to the splendor and the beauty of the star-spangled 
vault of heaven and to tell them that in the science of astronomy 
human reason triumphantly asserts its dominion over space and 
matter. For, relying on the universality of the divinely consti- 
tuted laws of nature, the astronomer weighs distant systems as 
on a scale, he predicts their future positions and configurations, 
and, aye, though trillions of miles away, he analyzes their compo- 
sition and determines the elements of which they are composed. 

In our commercial age where the unselfish, the good, and the 
noble are too often lost sight of in the pursuit of selfish and utili- 
tarian ideals a little book may well be in place which directs the 
gaze to the heavens of which the Psalmist sings : * * The heavens 
shew forth the glory of God and the firmament declareth the work 
of his hands.'' (Ps. XVIII, 2, 3.) 

St. John's University, 

Collegeville, Minn., April, 1905. 



CONTENTS. 

PAGE 

Introduction 1 

Laws of Motion. Apparent Motions of the 
Planets. Gravitation. The Telescope. The 
Spectroscope. 

Chapter I. Astronomy 7 

Sub-divisions of Astronomy. Its practical 
value. Its value as a mental discipline. 

Chapter II. History of Astronomy .... 8 

The Antiquity of Astronomy. Ancient Astrono- 
mers. Modern Astronomers. 

Chapter III. The Solar System 12 

Its various members. Its motion. 

Chapter IV. The Sun 13 

A hot, self-luminous body. Velocity of light. 
Distance and dimensions of the sun. 
Rotation. The different layers which 
envelope the sun's interior. Sunspots. 
Faculae. Elements in the sun. Tempera- 
ture. Apparent Motions of the sun. 
Exercise. 

Chapter V. Mercury 17 

Distance. Period of Revolution. Orbital 
velocity. Light and heat received from 
the sun. Variation of light and heat 
according to the law of inverse squares. 
Dimensions of Mercury. Rotation. Phases. 
Apparent motion. Transits. Exercise. 

Chapter VI. Venus 21 

Distance, etc. Morning and Evening star. 
Transits. Exercise. 



vi CONTENTS. 



Chapter VII. The Earth 24 

Distance from the sun. Period of revolution. 
Ecliptic. Orbital Velocity. Size and shape 
of the earth. Mass of the earth. Rota- 
tion. The seasons. Exercise. 

Chapter VIII. The Moon 29 

Nearest neighbor. Distance. Period of revo- 
lution. Orbital velocity. Dimensions. 
Rotates once a month. Fine object in the 
telescope. Phases. Harvest moon. Exer- 
cise. 

Chapter IX. Eclipses . . 33 

Eclipses of the sun ; total, partial and annular. 
Eclipses of the moon. When do eclipses 
occur ? The Saros. Exercise. 

Chapter X. The Tides ....... 38 

Different tides. Use of tides. Causes of the 
tides. Direction of travel of the tides. 

Chapter XL Mars. The Satellites of Mars . c 41 
Distance, period and orbital velocity of Mars. 
Light and heat from the sun. Dimensions. 
Rotation. Detail observed on Mars. Vary- 
ing brightness due to distance. Hall dis- 
covered the two moons of Mars. Distances 
and orbital velocity. Exercise. 

Chapter XII. The Asteroids 46 

What are they ? Dates of discovery of the 
first asteroids. Total number of the 
asteroids. Eros. Its importance to as- 
tronomy. How the asteroids are dis- 
covered. 

Chapter XIII. Jupiter 49 

Distance, etc. Peculiarity of rotation. Appear- 
ance in the telescope. Apparent motion. 
Exercise. 



CONTENTS. vii 



Chapter XIV. Jupiter's Satellites .... 53 

Number of Satellites. Their distances and 
periods. Barnard discovered the fifth satel- 
lite. Rotation. Eclipses. Perrine dis- 
covers two new satellites of Jupiter. 
Exercise. 

Chapter XV. Saturn . . 56 

Distance, etc. Unique object in the telescope. 
Dimensions of the rings. Exercise. 

Chapter XVI. The Satellites of Saturn ... 61 
Number, names, distances and periods. 
Phoebe. Discovered by W. Pickering. 
Its motion is retrograde. 

Chapter XVII. Uranus. The Satellites of Uranus . 65 
By whom discovered. Distance, etc. Uranus 
has four satellites. Names, distances and ^ 
periods. Revolution is retrograde. 

Chapter XVIII. Neptune. The Satellite of Neptune. 68 
Discovery of Neptune. Distance, etc. Its 
satellite's motion is retrograde. 

Chapter XIX. Comets 71 

Their appearance. Shape of a comet's orbit. 
Direction of the tail. Dimensions of 
comets. Their mass. Number and desig- 
nation of comets. 

Chapter XX. Meteors and Shooting Stars . • . 76 
Definition of a meteor. Their composition. 
Length of path and velocity. Total number 
of shooting stars per day. Velocity and 
mass. Meteoric shower. Radiant. Mete- 
oric swarms. Exercise. 

Chapter XXI. The Aurora Borealis and the Zodiacal 

Light 82 

Form of the Aurora. Time of occurrence and 
frequency. Zodiacal light best seen in 
Spring and Autumn. The * *Gegenschein. " 



viii CONTENTS. 



Chapter XXIL The Fixed Stars . ... . .85 

Number of fixed stars visible. Their nature. 
Distances. The light-year. Motion of the 
fixed stars. Magnitudes. Designation. 

Chapter XXIII. The Constellations .... 88 
How many are recognized. New constella- 
tions. Circumpolar constellations. The 
Zodiac. Constellation study. Location of 
the poles. 

Chapter XXIV. Northern Circumpolar Constellations. 90 

Chapter XXV. Equatorial and Adjoining Constella- 
tions 92 

Chapter XXVI. Southern Circumpolar and Neighbor- 
ing Constellations 105 

Exercise. 

Chapter XXVII. Temporary Stars and Variable Stars 109 
Number and possible causes of temporary 
stars. Classification of variable stars. 
Algol. Photography employed in the dis- 
covery of variable stars. Exercise. 

Chapter XXVIII. Double and Multiple Stars . . 113 
Naked-eye doubles. Stars optically and physi- 
cally double. Binaries. Their periods. 
Spectroscopic binaries. Multiple stars. 
Color contrasts. 

Chapter XXIX. The Galaxy, Star Clusters and Nebu- 
lae 117 

The galaxy is a vast throng of faint stars. Distri- 
bution of the stars. Naked-eye star clusters. 
The number and distribution of the nebulae. 
Notable nebulae. Exercise. 

Chapter XXX. The Nebular Theory . . . .119 

Appendix I. A Few Simple Problems on the Celestial 

Globe 122 

Appendix II. Astronomical Terms .... 125 



Catechism of Astronomy. 



INTRODUCTION. 



Laws of Motion, Gravitation, Telescope and 
Spectroscope. 

Motion. 

Q. 1. What is motion? 

A. Motion is a continuous and successive change 
of position. 

Q. 2. What is the simplest kind of motion ? 
A. The simplest kind of motion is motion along a 
straight line with uncnanging speed. 

Q. 3. What produces, changes or destroys motion 
in a body ? 

A. A force only, acting on a body, can produce, 
change, or destroy motion in that body ? 

Q. 4. If a body is at rest and no force acts on it, 
how will it behave ? 
A. It will remain at rest forever. 



INTRODUCTION. 



Q. 5. If a body is in motion and no force acts on 
it, how will it behave ? 

A. It will move on forever in a straight line and 
with constant speed. 

Q. 6. When a force acts on a body in the line of 
motion, how will it be affected ? 
* A. Its speed only will change. 

Q. 7. When a force acts continuously on a body 
across the line of motion, i. e,, at right angles with it, 
how will it be affected ? 

A. The speed of the body will remain unchanged 
but the direction will continuously change so that the 
body moves in a circular path. 

Q. 8. When a force acts on a body at an angle 
with the line of motion, how will it be affected ? 

A. In general, both the speed and the direction of 
motion will change. 

Q. 9. What is real m.otion of a body ? 
A. Real motion of a body is actual change of place 
of that body. 

Q. 10. What is apparent motion of a body? 

A. Apparent motion of a body is a seeming change 
of place of that body. In reality, the body is at rest 
and the observer moves in a direction just opposite to 
the apparent motion of the body. 

Q. 11. Can you give an example of this motion? 
A. When riding in a train, the trees, houses and 



INTRODUCTION. 



other objects near by seem to move in a contrary 
direction to that of the train. 

Apparent Motions of the Planets— Direct, Retrograde 
and Stationary. 

Q. 12. What is meant by the direct motion of a 
planet ? 

A. The motion of a planet is direct when its motion 
among the stars is eastward. 

Q. 13. When does this take place ? 
A. It takes place, when the planet is farthest from 
the earth. (Fig. 1, from P to P.) 




Fig. 1. Apparent Motion of a Planet. 
E represents the Earth, M a planet, and P, Pi, Pn, the apparent 
positions of this planet among the stars. 

Q. 14. What is the retrograde motion of a planet ? 
A. A planet has retrograde motion when its appar- 
ent motion among the stars is westward. 

Q. 15. When does a planet retrograde? 
A. It retrogrades when it is nearest to the earth? 
(Fig. 1, from P to P^\) 

Q. 16. When is a planet said to be stationary? 
A. A planet is said to be stationary when its 



INTRODUCTION. 



position among the stars remains seemingly fixed for 
some time. 

Q. 17. When does this occur? 
A. It occurs between the direct and retrograde 
motions of the planet. 

Q. 18. What is the cause of these appearances? 

A. The cause of these appearances is the combined 
motions of the earth and of the planet in their orbits 
around the sun. 

Gravitation. 

Q. 19. What is gravitation? 
A. Gravitation is that property common to all 
bodies whereby they attract each other. 

Q. 20. What is the law of gravitation ? 

A. Every particle of matter in the universe attracts 
every other particle with a force which acts along the 
line joining any two particles considered, and whose 
magnitude is proportionate directly to the product of 
the masses, and inversely to the square of the dis- 
tance between them. 

Q. 21. Who is the discoverer of this law? 
A. The great Sir Isaac Newton discovered this law 
toward the end of the seventeenth century. 

Q. 22. Is this law of any importance in astronomy? 

A. The law of gravitation is of the greatest im- 
portance in astronomy, for by it the motions of the 
heavenly bodies are controlled and their shapes 
determined. 



INTRODUCTION. 



The Telescope. 

Q. 23. What is a telescope ? 

A. A telescope is an optical instrument which 
magnifies the images of distant objects, or if the 
images be simply points of light, as those of the fixed 
stars, it increases their brilliancy. 

Q. 24. How many kinds of telescopes are there ? 
A. There are two kinds of telescopes, namely, the 
refractor and the reflector. 

Q. 25. Of what does the refractor in principle 
consist ? 

A. The refractor consists in principle of a light- 
gathering lens or system of lenses, called the object- 
glass, which makes the light-rays convergent, and of 
a second lens or system of lenses, called the eye-piece, 
which magnifies the image produced by the first. 

Q. 26. What are the essential parts of a reflector ? 

A. The essential parts of a reflector are a concave 
mirror which, after reflection, brings the light-rays, 
coming from a distant object, to a focus, and an eye- 
piece, as in the refractor. 

Q. 27. What other classification may be made of 
telescopes ? 

A. Telescopes are also classified as visual when the 
image is viewed with the eye, and as photographic, 
when the image is received on a photographic plate. 

Q. 28. Are telescopes important instruments to 
astronomy ? 

A. Telescopes are of the greatest importance to 



INTRODUCTION. 



astronomy, for they reveal a great many things which 
would otherwise remain hidden, e,g.,.the surface 
markings of the planets and millions of stars. 



The Spectroscope. 

Q. 29. What is a spectroscope ? 
A. A spectroscope is an optical instrument for 
forming and observing the spectra of bodies. 

Q. 30. What is a spectrum ? 
A. A spectrum is a series of images arranged 
according to wave-lengths. 

Q. 31. Have the elements spectra of their own? 
A. Yes, every element has a spectrum of its own. 

Q. 32. When a body is examined with a spectro- 
scope, what conclusions may be drawn ? 

A. The spectra show what elements are present in 
the body. 

Q. 33. How is the spectroscope used in astronomy? 

A. By examining with the spectroscope the light 
coming from the sun or a star, we learn what elements 
are present in them ? 

Q. 34. What other important thing does the 
spectroscope do ? 

A. By the shifting of the series of images (gen- 
erally they are lines or bands) in the spectrum, the 
spectroscope tells us how fast a star is approaching 
the earth or receding from it. 



ASTRONOMY. 



CHAPTER I. 



Astronomy. 

Q. 1. What is astronomy? 

A. Astronomy is the science which treats of the 
heavenly bodies, their motions, distances, magnitudes, 
physical condition and chemical composition. 

Q. 2. Has astronomy been subdivided? 

A. Yes, astronomy has a number of subdivisions, 
such as descriptive, practical, theoretical, mechanical, 
' spherical, and physical astronomy. 

Q. 3. On what division of astronomy is this cate- 
chism mostly based? 

A. This catechism is mostly based on descriptive 
astronomy which is nothing but a general statement 
of astronomical principles and facts. 

Q. 4. Is astronomy of any practical value? 
A. Yes. Geodesy, accurate surveying, navigation 
and time-service are all based on astronomy. 

Q. 5. Has astronomy any other value? 

A. Astronomy has very great value as a mental 
discipline; in fact, it forms an important part of a 
liberal education nowadays, and in the Middle Ages it 
was one of the seven liberal arts. 



HISTORY OF ASTRONOMY. 



CHAPTER II. 



History of Astronomy. 

Q. 1. Is astronomy of great antiquity? 

A. Astronomy was cultivated by the Chinese, 
Egyptians, Chaldeans, Greeks and Hindoos, even many 
centuries before the Christian era. 

Q. 2. Who were some of the most celebrated 
astronomers of antiquity? 

A. Pythagoras, Meton, Aristarchus, and Hippar- 
chus were some of the most celebrated astronomers of 
antiquity. 

Q. 3. What did Pythagoras teach? 

A. Pythagoras (B. C. 569-470) is said to have 
taught that there was a central fire in the universe 
around which the sun, moon, earth, planets and stars 
revolved. 

Q. 4. What did Meton discover? 

A. Meton (B. C. 433) discovered the Metonic cycle 
which consists of 235 synodic months (from new moon 
to new moon) and is very nearly equal to 19 common 
years of 365i days. 

Q. 5. For what was this cycle used? 

A. It was used to make the lunar years, according 
to which the ancient Greeks reckoned, correspond with 
the solar years; and it is still used in finding the time 
of Easter. 



HISTORY OF ASTRONOMY. 



Q. 6. What did Aristarchus maintain? 

A. Aristarchus (B. C. 310-250) was the first to 
maintain that the earth rotated on its axis and re- 
volved around the sun. 

Q. 7. What did Hipparchus do? 
A. Hipparchus (B. C. 190-120) ^^the Father of 
Astronomy'' was the first to make a catalogue of stars. 

Q. 8. Who were some of the renowned astro- 
nomers of the early Christian era? 

A. Ptolemy, Theon and his daughter Hypatia 
(375-415), and the Arabian astronomer Albategnius 
(877-929) are the best known astronomers of these 
times. 

Q. 9. What did Ptolemy write? 

A. Ptolemy (87-165), wrote a work which became 
known later by the name of Almagest, wherein he ex- 
plained his theory of the motions of the heavenly 
bodies. 

Q. 10. Was Ptolemy's theory widely accepted? 
A. Ptolemy's theory held full sway for 1400 years. 

Q. 11. Which were the salient features of the 
Ptolemaic system? 

A. According to this system the earth was at the 
center of the universe and did not move; the whole 
heavens revolved around the earth from east to west 
in one day; besides, the sun, moon, and planets were 
supposed to have certain proper motions to account 
for their apparent motions in the heavens. 



10 HISTORY OF ASTRONOMY. 

Q. 12. What prominent astronomers lived in later 
centuries? 

A. Regiomontanus, Copernicus, Tycho Brahe, Gali- 
leo, and Kepler. 

Q. 13. Who was Regiomontanus? 

A. Regiomontanus (1436-1476), whose real name 
was John Mueller, became the assistant of the astron- 
omer Purbach, at the age of sixteen; subsequently he 
calculated and published the places of the planets for 
many years ahead. 

Q. 14. Who was Copernicus? 

A. Copernicus (1473-1543), a canon at the cathedral 
of Frauenburg, is the author of the system which bears 
his name. 

Q. 15. What is the Copernican system? 

A. The Copernican system with a few later cor- 
rections and additions is the astronomical system 
which is now universally held and which is known to 
be the only true system, 

Q. 16. What are the main points of the Copernican 
system? 

A. It teaches that the earth rotates on its axis and 
with the other planets revolves around the sun. 

Q. 17. Who was Tycho Brahe? 

A. Tycho Brahe (1546-1601), was the best observer 
of his tim^; he also devised an astronomical system 
which, however, was never widely accepted. 

Q. 18. Who was the first to use the telescope for 
astronomy? 



HISTORY OF ASTRONOMY, 11 

A. Galileo (1564-1642), was the first to apply the 
telescope to astronomical observation. 

Q. 19. What did Kepler discover? 

A. Kepler (1571-1630) by careful comparison and 
study of the recorded positions of the planets, espec- 
ially those made by Tycho Brahe, discovered the three 
great physical laws which bear his name. 

Q. 20. Which is the first of Kepler's Laws? 
A. The first law states that the orbit of each planet 
is an ellipse, with the sun in one of its foci. 

Q. 21. Who are some noted astronomers of still 
later times ? 

A. Huyghens, Roemer, Newton, and William 
Herschel. 

Q. 22. What is to be noted of Huyghens ? 
A. Huyghens (1629-1695) proposed the wave-theory 
of light, and made the first pendulum clock. 

Q. 23. Who was Roemer ? 

A. Roemer, a Dane (1644-1710) is the inventor of 
the transit instrument; he likewise roughly deter- 
mined the velocity of light. 

Q. 24. What is to be said of Newton ? 

A. Newton (1642-1727) discovered the law of 
universal gravitation and wrote a monumental work 
called the Principia, 

Q. 25. What did William Herschel do ? 

A. William Herschel (1732-1832) built several large 
reflecting telescopes; he also discovered the planet 
Uranus. 



12 



THE SOLAR SYSTEM. 



Q. 26. Are there noted astronomers who lived 
later than the ones just mentioned ? 

A. Yes, there are a great many noted astronomers 
who lived after HerscheFs time, or who are still living; 
some of their discoveries will find mention in the 
different chapters of this catechism. 



CHAPTER III. 



The Solar System. 

Q. 1. Of what does the solar system consist ? 
A. The solar system consists of the sun and all the 
bodies that revolve around it. 




Fig. 2. Solar System. 

Q. 2. What bodies are these ? 

A. They are the planets and their satellites, 
asteroids, comets and meteorites. 

Q. 3. Which are the planets in order of distance 
from the sun ? 



THE SUN. 13 



A. They are Mercury, Venus, Earth, Mars, Jupiter, 
Saturn, Uranus, and Neptune. 

Q. 4. Which are the satelHtes ? 
A. The earth has one moon, Mars two, Jupiter 
seven, Saturn ten, Uranus four, and Neptune one. 

Q. 5. Where are the asteroids ? 
A. The asteroids which are small planet-like bodies 
have their orbits between those of Mars and Jupiter. 

Q. 6. Are all comets permanent members of the 
solar system ? 

A. No, most comets are only visitors to our system 
and most probably will never return to it. 

Q. 7. What paths are many meteorites known to 
follow ? 

A. Many meteoric swarms are known to follow 
along the paths of certain comets. 

Q. 8. Is the solar system stationary in the 
universe ? 

A. No, the sun with all its dependent bodies is 
moving towards a point in the constellation of Her- 
cules with a velocity estimated at from eleven to 
fifteen miles a second. 



CHAPTER IV. 



The Sun. 

Q. 1. What is the sun? 

A. The sun is an intensely hot, self-luminous globe 
around which the planets circle and from which they 
receive nearly all their light and heat. 



14 THE SUN. 



Q. 2. How does the light and heat of the sun 
reach the planets ? 

A. They reach it in form of rapid vibration caused 
by the sun in that substance which is believed to per- 
vade the whole universe and which has been named 
the luminif erous ether. 

Q. 3. How fast does light travel through space ? 
A. Light travels with a velocity of 186,330 miles a 
second. 

Q. 4. What causes the planets to circle around 
the sun ? 
A. It is the sun's attraction. 

Q. 5. If the law of gravitation according to which 
bodies attract each other would suddenly cease to 
exist, how would the planets then move ? 

A. They would move off into space along a straight 
line tangent to the curve at that particular moment 
and with the speed with which they were then mov- 
ing. A planet at P would move off towards T, 




Fig. 3. Were Gravitation to Cease. 

Q. 6. How far is the sun. distant from the earth? 



THE SUN. 15 



A. The mean distance of the sun from the earth 
is 92,900,000 miles. 

Q. 7. What is the length of the sun's diameter? 

A. The sun's diameter is 866,500 miles, which is 
109.5 times the diameter of the earth. 

Q. 8. How great is the sun's surface ? 

A. The sun's surface is about 12,000 times as great 
as the surface of the earth.* 

Q. 9. How great is the sun's volume ? 

A. The sun's volume is about 1,300,000 times that 
of the earth, t 

Q. 10. What is the sun's mass ? 

A. The sun's mass is very nearly 332,000 times the 
mass of the earth. 

Q. 11. How much would a body, weighing a pound 
here on earth, weigh on the sun ? 

A. The body would weigh 27.6 pounds ; a person 
weighing 150 pounds on earth, would weigh 4,140 
pounds (over two tons) on the sun. 

Q. 12. Does the sun rotate on its axis ? 

A. Yes, the sun rotates on its axis. 

Q. 13. How long does it take the sun to turn 
around once ? 

A. At the equator it completes a rotation in 25 
days ; at solar latitude 20"", in 25.75 days ; at solar 
latitude 40°, in 27 days. 

Q. 14. What does this show ? 

A. It shows that the sun's surface is not solid, 

* Surfaces of globes are to each other as the squares of their 
diameters; in this case, (109. 5) ^i 12 

t Volumes of globes are* to each other as the cubes of their 
diameters; in this case, (109,5)3: 13 



16 THE SUN. 



otherwise the time of rotation would have to be the 
same for all latitudes. 

Q. 15. Is the interior of the sun solid? 

A. Nothing is known of the sun's interior, except 
by inference, but it seems quite certain that the sun 
is still in the gaseous state. 

Q. 16. How have astronomers named the successive 
layers of the sun that can be observed? 

A. The luminous surface of the sun has been called 
the photosphere, above this lies the chromosphere from 
which the prominences rise, and surrounding it all is 
the corona. 

Q. 17. When only, can the corona be seen? 

A. The beautiful corona can only be seen at a total 
eclipse of the sun. 

Q. 18. What are sunspots? 

A. Sunspots are dark depressions in the photo- 
sphere which remain for some time and then disappear. 

Q. 19. What is the nature of these spots? 

A. The nature of these spots is not yet thoroughly 
understood; it seems quite probable however, that the 
spots are caused by cooled vapors falling back to the 
sun. 

Q. 20. What are faculae? 

A. Faculae are brilliant white patches on the 
photosphere, generally seen in the neighborhood of 
sunspots; they are thought to be heated vapors rising 
violently from the sun's interior. 

Q. 21. Does the spectroscope reveal anything re- 
garding the chemical constituents of the sun ? 



M 



.3^ f- 






^-.^ >*^ 



Fig. 4. Sun Spots and Faculae at the Time of a Spot 
Maximum. 



MERCURY. 17 



A. Yes, the spectroscope shows that the sun con- 
tains many elements which are present in the earth, 
such as iron, hydrogen, sodium, nickel, carbon, 
copper, etc. 

Q. 22. How hot is the sun ? 

A. The effective temperature of the sun is very 
likely somewhere between 10,000° and 20,000° F. 

Q. 23. Has the sun any apparent motions ? 

A. Yes, the sun has two apparent motions; viz: 
a diurnal motion from east to west and an annual 
motion around the ecliptic from west to east. 

Q. 24. What causes the diurnal motion? 

A. The earth's rotation from west to east. 

Q. 25. What causes the annual motion ? 

A. The earth's revolution around the sun. 

Exercise.— By means of the star-maps determine what con- 
stellations are on the meridian, that is, on the north and south line 
in the sky, about 2 hours after sunset. Write your observation in 
a memorandum book. After a month make a similar observation. 
You will notice that those stars which were on the meridian at the 
first observation are now to the west of it; in other words, the sun 
has moved towards them. If you repeat these observations for a 
year, you will find that the sun has made a complete circuit of the 
heavens. What did really move? the sun, or the earth? 



CHAPTER V. 



Mercury. ^ 
Q. 1. What planet is nearest to the sun? 
A. Mercury is the planet which is nearest to the 
sun. 

Q. 2. How far is it distant from the sun? 
A. The mean distance of Mercury is 36,000,000 
miles; its actual distance varies from 28,500,000 miles 
43,500,000 miles. 



13 MERCURY. 



Q. 3. In what time does Mercury revolve around 
the sun? 

A. Mercury completes its revolution around the 
sun in very nearly 88 days. 

Q. 4. How fast does this planet move along in its 
orbit? 

A. When nearest the sun it has a speed of 35 miles 
a second, which decreases to 23 miles a second when 
it is at that part of its orbit which is farthest away 
from the sun. 

Q. 5. How much light and heat does Mercury 
receive from the sun? 

A. The heat and light of the sun is 6.7 times more 
intense at Mercury than at the distance of the earth. 

Q. 6. How does the intensity of light and heat 
vary? 

A. The intensity of light and heat varies inversely 
as the square of the distance from the body emitting 
the light and heat. (In Fig. 5, the light passing 
through 1 is spread over 2, which has four times the 
area of 1.) 




Fig. 5. Light and Radiant Heat Vary Inversely as the 
Square of the Distance. 



MERCURY. 19 



Q. 7. How may this be shown for Mercury? 

A. If we call the distance of the earth from the 
sun one, then the distance of Mercury from the sun 
is expressed by Iff i]f Sf S- = 0.387. Therefore, light 
at Mercury: light at earth = (oItP • ~i^' Hence, light 
at Mercury = iq-Aw^ X light at earth = 6.7 X light at 
earth. 

Q. 8. What is the diameter of Mercury ? 

A. Mercury's diameter is very near 3,000 miles. 

Q. 9. How does the surface and volume of this 
planet compare with that of the earth? 

A. Its surface is |, and its volume il^ of the 
earth's. 

Q. lO. Does Mercury rotate on its axis ? 

A. It seems to be quite certain now that Mercury 
turns just once on its axis in the time it goes around 
the sun. 

Fig. 6.— Phases of Mercury and Venus. 

Q. 11. What is the consequence of this ? 
A. A consequence of this is that one side of 
Mercury has perpetual night. 



20 MERCURY. 



Q. 12. How does Mercury appear in a telescope ? 

A. In a telescope Mercury appears like a little 
moon, and shows all the phases like our own moon. 
(See Fig. 6.) 

Q. 13. What do the phases of Mercury prove ? 
A. They prove that the planet shines by the 
reflected light of the sun. 

Q. 14. How does Mercury appear to the naked 
eye? 

A. When Mercury is far enough away from the 
sun to be seen in the twilight, it appears like a 
brilliant star of the first magnitude. 

Q. 15. What is the apparent motion of this 
planet ? 

A. It never moves more than 28° away from the 
sun ; it moves east from the sun and can then be 
seen in the evening twilight; then it moves west- 
ward, passes the sun and swings out west of it, and 
can then be seen in the morning twilight ; thereupon 
it resumes its eastward motion. 

Q. 16. What is a transit of Mercury ? 

A. Sometimes Mercury passes exactly between 
the sun and earth ; when this takes place Mercury 
can be seen with a telescope as a dark round spot 
crossing the disk of the sun. This phenomenon is 
called a transit. 

Q. 17. When will the next three transits occur? 
A. They will occur: November 12, 1907, 
November 6, 1914, and May 7, 1924. 



VENUS. 21 



Exercise. — From a calendar which gives astronomical data 
determine at what time an eastern elongation of Mercury occurs. 
When one occurs in March or April, take that one. Commence 
looking for a bright star near the western horizon on about the fifth 
or sixth evening before elongation. By means of the star-maps 
make sure that what you suppose is Mercury is not a fixed star. 
If you have a telescope wherewith to view the star, the presence 
or absence of the phase, a little more than half full, will tell if it 
is Mercury or not. Observe the planet every clear evening 
through its elongation. Note also, if possible, its motion among 
the stars until it is again lost in about fourteen days in the sun's 
glare. With the telescope the change from gibbous to cresent 
phase could be observed. 



CHAPTER VL 



Venus. 



Q. 1. What is Venus ? 

A. Venus is the planet whose orbit is between the 
orbits of Mercury and the earth ; it is the most 
brilHant of all the planets, and is at times easily 
seen during the day with the naked eye. 

Q. 2. How far is Venus distant from the sun ? 

A. The mean distance of Venus from the sun is 
67,200,000 miles ; its greatest and least distances do 
not differ more than 470,000 miles each way from the 
mean ; the orbit, therefore, in which Venus revolves 
around the sun is almost a circle. 

Q. 3. In what time does Venus complete a 
revolution around the sun ? 

A. Venus completes a revolution around the sun 
in 225 days. 

Q. 4. How fast does this planet travel in its orbit? 
A. It has an orbital velocity of 22 miles a second. 



22 VENUS. 



Q. 5. How much light and heat does Venus 
receive from the sun ? 

A. Venus receives 1.9 times as much Hght and 
heat from the sun per unit area as the earth. 

Q. 6. What is the diameter of Venus ? 

A. Its diameter is 7,700 miles. 

Q. 7. What is the surface and volume of this 
planet ? 

A. Its surface is 95 per cent of the earth's 
surface, and its volume is 92 per cent. 

Q. 8, What is the rotation period of Venus ? 

A. Formerly it was believed to be nearly 24 
hours ; but the observations of Schiaparelli, Lowell, 
and others, make it very probable that its period of 
rotation is of the same length as its period of 
revolution, so that Venus, like Mercury, always keeps 
the same side turned towards the sun. 

Q. 9. How does Venus appear in the telescope ? 

A. On account of her great brilliancy, Venus is a 
striking object in the telescope ; she shows all the 
phases of the moon and at certain parts of her orbit 
even a moderate magnifying power makes her 
appear as large as our own moon. (See Fig. 5.) 

Q. 10. Who was the first to discover the phases 
of Venus ? 

A. Galileo discovered the phases of Venus in 
1610 ; this discovery furnished a strong proof for the 
theory of Copernicus. 

Q. 11. Is Venus easily seen without instrumental 
aid? 



VENUS. 23 



A. When not too near to the sun Venus is easily- 
seen, for this planet is by excellence the Morning 
and the Evening Star. 

Q. 12. What are the apparent motions of the 
planet Venus ? 

A. Venus is, like Mercury, a close attendant on 
the sun and never departs more than 48° degrees 
from the Sun ; when it moves east of the sun it is 
the beautiful Evening Star in the western sky, and 
when it moves west and passes the sun on its way it 
becomes the Morning Star. 

Q. 13. What is a transit of Venus? 

A. When Venus passes exactly between the earth 
and the sun it is then visible even without a telescope 
as a black spot which slowly moves across the sun's 
disc from east to west ; this occurrence is called a 
transit. 

Q. 14. Are transits of Venus numerous ? 

A. These transits occur but seldom ; the last 
transit was December 6, 1882, and the next one will 
be June 8, 2004. 

Q. 15. Are the transits of Venus of any im- 
portance in astronomy ? 

A. They were of great importance as furnishing 
a method of determining more closely the sun's 
parallax, and the distance of the earth from the sun; 
more exact methods have since been used to 
determine these important constants. 

Q. 16. What made the transit-method of finding 
the sun's parallax less exact than was first expected ? 



24 THE EARTH. 



A. The atmosphere of Venus which caused it to 
be surrounded by a ring of hght, and a certain 
phenomenon of refraction which makes a sharp 
separation between Hght and darkness impossible. 

Q. 17. How high is the atmosphere of Venus ? 
A. Astronomers judge the atmosphere of Venus 
to be about 55 miles high. 

Exercise.— From a calendar determine when Venus becomes an 
evening star. A few months after the given date begin to look 
for a very bright star low down in the western horizon when the 
twilight begins to fade. You will notice that week after week 
the planet slowly increases its distance from the sun. After 220 
days, counting from the time when Venus passed the sun on the 
eastward swing, it has attained greatest elongation, about 47^. 
After a moment's halt it begins to move westward and reaches 
the sun in about 72 days. Venus now becomes a morning star, 
and in 72 more days has reached its point farthest west from the 
sun. After 220 more days the planet is back to the position with 
reference to the sun and the earth from where we supposed it to 
start. It takes Venus, therefore, 585 days to pass through its 
cycle of apparent motions ; this is known as the synodic period of 
Venus. Similarly, the synodic period of Mercury is about 116 
days. Can you give the reason why the time of the westward 
swing is so much shorter than that of the eastward swing ? 



CHAPTER VIL 



The Earth. © 

Q. 1. What is the earth ? 

A. The earth is the planet which is third in the 
order of distances from the sun. 

Q. 2. How far is the earth away from the sun ? 

A. The mean distance of the earth from the sun 
is 92,900,000 miles ; the actual distance varies about 
1,500,000 miles each way from the mean distance. 

Q. 3. When is the earth nearest to the sun ? 



THE EARTH. 25 



A. The earth is nearest to the sun, or in perihelion, 
about the second of January. 

Q. 4. When is it farthest away? 
A. It is farthest away, or in aphelion, six months 
later. 

Q. 5. In what time does the earth go around the 
sun ? 

A. The earth goes around the sun in 365i days 
nearly. 

Q 6. Why are there leap-years ? 
A. To make up for the i days which are dropped 
in ordinary years. 

Q. 7. What is the plane in which the orbit of the 
earth lies, called ? 
A. It is called the ecliptic. 

Q. 8. Do the orbits of the other planets also lie in 
the ecliptic ? 

A. No ; none of the orbits of the other planets lie 
in the ecliptic, but they are all slightly inclined to it; 
Mercury's orbit makes the largest angle, 7°, and the 
orbit of Uranus the smallest angle, a little more than 
i of a degree, with the ecliptic. 

Q. 9. How fast does the earth travel through 
space in its orbit around the sun ? 

A. The earth travels with a velocity of 18.5 miles 
a second in its orbit ; this exceeds the velocity of 
a cannon-ball about seventy-five times. 

Q. 10. How much heat does the earth receive 
from the sun ? 



26 THE EARTH. 



A. If the sun's heat could be distributed evenly 
over the earth's surface it would melt in one year a 
layer of ice spread over the whole earth and having a 
thickness of 177 feet. 

Q. 11. How long does it take the sun's light and 
heat to reach the earth ? 

A. The sun's light and heat reach the earth in 8 
minutes and 19 seconds. 

Q. 12. What is the shape of the earth ? 

A. The earth is nearly a globe in shape; the other 
planets, their satellites and the asteroids all have this 
shape. 

Q. 13. What causes these bodies to have the glob- 
ular form? 

A. These bodies are so formed according to the 
law of gravition. 

Q. 14. What is the length of the earth's diameter ? 

A. The equatorial diameter of the earth is 7927 
miles, and the polar diameter is 7900 miles. 

Q. 15. Does this polar flattening affect the weighi 
of bodies on the earth ? 

A. Yes, the polar flattening, together with the 
greater speed of rotation at the equator, cause bodies 
to weigh more (by a spring balance) when they are 
transferred towards the pole, although their mass re- 
mains the same. 

Q. 16. How great is this difference ? 

A. It is one part in one hundred and ninety; a per- 
son weighing 190 pounds at the equator would weigh 
191 pounds at the north pole. 



THE EARTH. 27 



Q. 17. What is the surface of the earth ? 
A. The earth's surface is nearly 197,000,000, 
square miles. 

Q. 18. What is the volume of the earth ? 
A. The earth's volume, or solid contents, is about 
260,000,000,000 cubic miles. 

Q. 19. How heavy is the earth ? 

A. If the earth could be placed on a suitable bal- 
ance it would weigh six sextillions of tons; this is 
about 5.5 times as much as if it were made entirely of 
water. 

Q. 20. In what time does the earth turn on its 
axis? 

A. It turns on its axis once in 24 sidereal hours, or 
in 23 hours 56 minutes 4 seconds of ordinary time. 

Q. 21 What does this rotation cause ? 
A. This rotation causes the succession of day and 
night. 

Q. 22. How often does the earth turn on its axis 
in a year of 365 days ? 

A. It turns 366 times on its axis. i. e., any fixed 
star will pass the north and south line (meridian) of 
an observer 366 times in a year of 365 days. 

Q. 23. What causes this difference of one ? 
A. The earth's revolution around the sun. 

Q. 24. What produces the seasons ? 
A. The season's are due to two things, viz: the 
annual motion of the earth around the sun, and the 



28 



THE EARTH. 



constant direction and inclination of the earth's axis 
of rotation to the plane of the ecliptic. (See Fig. 7) 



y 



^k\^ 'autumnal equinox 



^^IBWtEfi Soisifct 



fSRIHELION 




UMM£B SOLSTICE 



^^^ VERNAL EQUINOX 

Fig. 7— The Seasons. 

Q. 25. How great is this inclination ? 

A. The earth's axis is inclined to the pole of the eclip- 
tic by an angle of a little less than 23i°; i. e., the equator 
makes this same angle with the ecliptic. 

Q. 26. What apparent motions do the actual 
motions of the earth produce ? 

A. The daily rotation of the earth from west to 
east produces the apparent daily rotation of the 
heavenly bodies from east to west; and the annual 
revolution of the earth produces the apparent motion 
of the sun around the ecliptic, its apparent motion 
being always exactly opposite to the real motion of 
the earth. 

Exercise. You are all, no doubt, familiar with the proofs 
which your geographies give for the ball-like shape of the earth. 
Perhaps you do not live near the big ocean or near a great lake 
where one can see the hulls of the ships hide behind the earth; and 
still you would like to see with your own eyes that the earth is 



THE MOON. 29 



round. This is not so difficult, any lake, a mile across and even 
less will show it. Place a bright object (a piece of shiny tin, for 
instance) on one side of the lake and let it stand a few inches 
above the water. Then go to the other side and see how close you 
must bring your eye to the water in order to make the bright object 
disappear behind the curve of the lake's surface. The lake must, 
of course, be calm. In regions where the ponds are covered with 
ice in winter, the experiment is very easily made when the lake 
is smoothly frozen over. The curvature of the earth is about 
8 inches to the mile. Since this curving of the surface of extended 
bodies of water is noticed in all regions of the earth, it follows 
that the earth must have a globular form. 



CHAPTER VIII. 



The Moon. ^ 

Q. 1. What is the moon? 

A. The moon is our nearest celestial neighbor; it 
accompanies the earth on its journey around the sun, 
and it moves at the same time in an elliptical orbit 
around the earth. 

Q. 2. How far distant is the moon from the earth? 

A. The moon's mean distance from the centre of 
the earth is 238,800 miles; its extreme distances are 
253,000 and 221,600 miles. 

Q. 3. In what time does the moon complete its 
revolution around the earth? 

A. The moon completes its revolution around the 
earth in 27 days, 7 hours 43 minutes; this is the 
average period; different revolutions may differ by 
several hours. 

Q. 4. What is this revolution called? 

A. It is called the sidereal revolution of the moon. 

Q. 5. What is the synodic revolution of the moon? 



30 THE MOON. 



A. The synodic revolution of the moon is the time 
from new moon to new moon again, or from full moon 
to full moon; its average length is 29 days, 12 hours, 
44 minutes. 

Q. 6. What causes this difference in time between 
the sidereal and the synodic revolution? 
A. The change of position of the earth in her orbit. 

FULL MOON 




FULL MOON 

Fig. 8. Sidereal and Synodic Months. 

Q. 7. At what rate does the moon travel around 
the earth? 

A. Besides flying through space with the earth in 
the yearly orbit at the rate of 18.5 miles a second, the 
moon travels in its orbit around the earth at the rate 
of 2288 miles per hour. 

Q. 8. What is the diameter of the moon? 
A. The moon's diameter is 2160 miles. (See 
figure 9.) 

Q. 9. How does the moon's surface and volume 
compare with the surface and volume of the earth? 

A. The moon's surface is somewhat more than one 
fourteenth of the earth's surface; and its volume very 
nearly one forty-ninth of the earth's volume. 

Q. 10. Does the moon rotate on its axis? 




Fig. 9. Moon Seven Days Old. 




Fig. 10. Moon's Age Twenty-one Days, Five Hours. 



THE MOON. 31 



A. Yes, the moon rotates once in a revolution, or 
sidereal month. 

Q. 11, What does this effect? 

A. It effects that the moon always presents the 
same side to the earth, and also that on the moon the 
day and the night is each roughly two weeks long. 

Q. 12. Is the moon a fine object in the telescope? 

A. On account of its nearness, the telescope shows 
comparatively smooth stretches of surface, mountain 
ranges, gigantic, crater-like formations and craterlets, 
rills, rays, etc. (See figure 10.) 

Q. 13. Is there any air or water on the moon? 
A. The moon has very little if any atmosphere, and 
no water. 

Q. 14. What is meant by the moon's phases? 

A. The moon's phases are the. various appearances 
of the moon which are due to those portions of the 
illuminated half of the moon which are turned toward 
the earth. 

Q. 15. How do they succeed each other? 

A. At new moon, the moon stands between the 
sun and the earth, so that the illuminated half of the 
moon is entirely turned away from us; then the 
moon moves east of the sun in the sky and a slender 
cresent becomes visible of the illuminated half 
of the moon; the farther the moon moves away 
from the sun, the more becomes visible of the illumi- 
nated side, arid so the crescent grows to first 
quarter, then to gibbous, and finally to full moon 
phase; at this stage the earth stands between the 



32 THE MOON. 



moon and the sun; hereupon the mocn approaches 
the sun again from the west and the phases become 
successively gibbous, last quarter, crescent, and finally, 
new moon phase. (See Fig. 11) 




Fig. H-Phases Of The Moon. 
The Sun is supposed to be off to the right. A is new moon, B is 
the crescent visible in the evening", C is first quarter, D is the gib- 
bons phase, E is full moon, F is the gibbons phase after full moon, 
G is last quarter, and H is the crescent visible in the morning sky. 

Q. 16 What do the phases prove ? 

A. They prove that the moon is a dark body and 
shines by reflected light only. 

Q. 17. What is the harvest moon ? 

A. The harvest moon is the full moon which occurs 
nearest to the autumnal equinox (about September 22). 

Q. 18. What is remarkable about the harvest 
moon? 

A. At the harvest moon, the moon rises for a num- 



ECLIPSES. 33 

ber of evenings at nearly the same time, thus giving us 
a series of splendid moonlit evenings. 

Q. 19. What is the cause of this ? 

A. The portion of the ecliptic where this full moon 
occurs is then least inclined to the horizon and so the 
moon, as it were, coasts along the horizon; since, how- 
ever the moon's orbit is inclined to the ecliptic by 
about 5° and since the direction of this inclination 
continually changes, the effect is more marked in cer- 
tain years than in others. The next full moon shows 
similar phenomena and is known as the hunter's moon. 

EXERCISE. Note the moon's position among the stars a few 
days after new moon. Make a little map of the moon and the 
brighter stars near it. The following evening do the same and 
compare the two maps. The distance between the two positions 
will be around 13°. At that rate, how many days would it require 
to make the whole circle of 360° ? Examine if there are any 
bright stars in the moon's path to the east of the moon. If there is 
one quite close to the moon, observe how the moon passes in front of 
the star and hides it from our view. This phenomenon is called 
an occultation. The disappearance is instantaneous, and affords one 
of the best proofs that the moon has no atmosphere, or, at most, 
an extremely rare one. ^ An opera glass will be of great help in 
observing this very striking and interesting phenomenon. 



CHAPTER IX. 



Eclipses. 
Q. 1. What is an eclipse of the sun ? 
A. An eclipse of the sun is the partial or total 
hiding of the sun's disc by the moon. 

Q. 2. What is a total eclipse ? 
A. An eclipse of the sun is total when the sun is 
entirely hidden by the moon. (See Fig. 12) 

Q. 3. How long may a total eclipse last ? 

A. From first to last contact may take a little 



34 



ECLIPSES. 



more than four hours; the time of total obscuration 
can never exceed 7 minutes 58 seconds, and is usually 
much less. 




Fig. 12— Total Eclipse Of The Sun. 

Q. 4. Is a total eclipse of the sun visible in widely 
distant places ? 

A. A total eclipse is visible only within a very long 
but narrow strip of the earth's surface; the eclipse is 
however visible as a partial one to big distances on 
either side of the path of totality. 

Q. 5. What is a partial eclipse of the sun ? 

A. At a partial eclipse, the sun, moon, and earth 
do not get quite in line and the moon covers up the 
sun only partly. 

Q. 6. What is an annular eclipse ? 




Fig. 13. Annular Eclipse of the Sun. 

A. At an annular eclipse the moon is too far away 
from the earth so that its shadow cannot reach the 
earth; the moon appears therefore smaller than the 
sun and in passing in front of the sun will leave a 
ringlike portion of the sun unobscured. 

Q. 7. When does an eclipse of the moon occur? 



ECLIPSES. 35 



A. An eclipse of the moon takes place when 
the moon passes through the earth's shadow; in other 
words, when the earth stands between the sun and 
the moon. ( See Fig. 14.) 

Q. 8. When is a lunar eclipse total, and when 
partial ? 

A. It is total when the moon passes entirely into 
the earth's shadow, and partial, when the moon gets 
only partly within this shadow. 

Q. 9. How long can an eclipse of the moon last ? 

A. A total eclipse of the moon may last under best 
conditions about four hours from first to last contact; 
the time of total obscuration is about two hours. 




Fig. 14. Total Eclipse of the Moon. 

Q. 10. Is an eclipse of the moon widely visible ? 

A. Yes, an eclipse of the moon is visible from the 
half of the earth's surface which is turned toward 
the moon. 

Q. 11, At what time only, can eclipses occur? 
A. Eclipses of the sun can occur at new moon only, 
and eclipses of the moon occur only at full moon. 

Q. 12. Why are there not eclipses at every new 
moon and every full moon ? 

A. Because the moon's orbit is inclined to the 
ecliptic where the eclipses occur by an angle of over 



36 ECLIPSES. 



5°, hence the moon is but twice a month in the ecHp- 
tic, when it crosses it from above and from below; 
only when these points of crossing, called the nodes, 
lie in line with the sun and the earth do eclipses take 
place. 

Q. 13 Are eclipses of frequent occurrence ? 

A. Two eclipses, at least, must occur each year, 
and these would be eclipses of the sun; the greatest 
number that can take place in a year is seven, and of 
these five would be of the sun and two of the moon. 

Q. 14. Are eclipses of the sun more frequent than 
eclipses of the moon? 

A. Yes, there are about four solar eclipses to three 
lunar eclipses; the lunar eclipses are however more 
widely visible. 

Q. 15. WhatistheSaros? 

A. The Saros is a period of time equal to eighteen 
years and eleven and one-third days; if there should 
however be five leap years in this interval, then in- 
stead of eleven and one third, ten and one-third days 
are taken. 

Q. 16. What purpose does the Saros serve? 
A. The Saros has been used from remote antiquity 
on in foretelling eclipses. 

Q. 17. How is it applied ? 

A. If there was an eclipse of the sun or of the moon 
on a certain date, then eighteen years eleven and one- 
third days later a like eclipse will happen; in this 
manner an eclipse of the moon will be repeated nearly 



ECLIPSES. 37 



fifty times, and an eclipse of the sun will be repeated 
around seventy times. 

Q. 18. What is the cause of this recurrence? 

A. In the interval of a Saros the moon gets very 
nearly back to the position it occupied with reference 
to the sun and the nodes. 

Q. 19. When was the last total eclipse of the sun 
which was visible in the United States ? 

A. The last total solar eclipse visible in the United 
States was on May 28th, 1900. 

Q. 20. When will the next eclipses of the sun occur, 
which are total eclipses somewhere in the United 
States? 

A. They will occur in 1918, 1923 and 1925. 

Exercises. 

From a calendar find out if any of the eclipses which occur during 
the year are visible in the region where you live. If they be 
eclipses of the moon, no special preparations are necessary. Note 
at what side the shadow cast by the earth begins to encroach upon 
the lunar landscape. If it be a total eclipse, the moon will most pro- 
bably remain visible even at greatest totality ; but it will shine with a 
dull copper-colored light; this is light which is bent into the shadow 
by the earth's atmosphere. If they be eclipses (partial) of the 
sun, some sort of shade to reduce the brilliancy of the sun will be 
necessary. Light a small piece of camphor and hold a piece of an 
ordinary window-pane over the flame. The glass will soon be 
covered with a layer of soot through which one can easily view the 
sun. Note from what direction the moon encroaches upon the sun's 
disc. Can you explain from aknowlege of the moon's motion 
among the stars why it should be in that direction? If it be your 
good fortune to get within the path of totality of a total solar 
eclipse, you should make beforehand a study of the phenomena 
attending a solar eclipse in order to view properly what is one of 
the grandest of natural events. 



38 THE TIDES. 



CHAPTER X. 



The Tides. 
Q. 1. What are the tides ? 

A. The tides are the periodical rising and falling 
of the water of the ocean. 

Q. 2. How often does the water of the ocean rise 
and fall ? 

A. The water rises and falls twice in the interval 
of a little more than a day ; the average interval 
being 24 hours 51 minutes. 

Q. 3. When is it flood tide ? 

A. It is flood tide when the water is rising. 

Q. 4. When is it ebb tide ? 

A. It is ebb tide when the water is falling. 

Q. 5. What are spring tides ? 

A. Spring tides are the highest tides of the month. 

Q. 6. When do they occur ? 

A. They occur at the time of new moon and full 
moon. 

Q. 7. What are neap tides. 

A. Neap tides are the smallest tides of the month. 

Q. 8. When do they occur ? 

A. They occur at the time of flrst quarter and last 
quarter of the moon. • 

Q. 9. How much higher, about, are the spring tides 
than the neap tides? 

A. The spring tides are about If times as high 
as the neap tides. 



THE TIDES. 39 



Q. 10. Are tides of any use ? 

A. Yes, at time of flood tide large ships can ascend 
shallow harbers that would otherwise be inaccessible 
to them. 

Q. 11. What causes the tides ? 

A. The attraction of the moon and the sun causes 
the tides; but more especially the attraction of the 
former. 





Fig. 15.— Spring Tide. 

Q. 12. Why is the tide-raising force of the moon 
greater than that of the sun ? 

A. Because the moon is so much nearer to the 
earth. 

Q. 13. What fraction of the moon's tide-raising 
force is the sun's tide-raising force ? 

A. It is about f of the moon's tide-raising force. 

Q. 14. How many flood tides are produced by the 
moon or by the sun at any one time ? 

A. There are always two tides produced by these 
bodies ; one is, in general, turned towards them, and 
the other is on the other side of the earth turned 
away from them. 

Q. 15. How are these double tides to be ex- 
plained ? 



40 THE TIDES. 



A. Since the tides are a consequence of the 
attraction of these bodies, and since the attraction 
varies inversely as the square of the distance, it fol- 
lows, that the part of the earth's surface turned 
toward the moon, for instance, is drawn more than 
the earth's centre, and that the earth's centre is 
drawn more than the surface turned farthest away ; 
hence the waters of the ocean will bulge out in 
these directions as much as their elasticity, weight 
and inertia will allow. 

Q. 16. Where are the ebb tides situated ? 
A. The ebb tides are situated half-away on either 
side between the flood tides. 

Q. 17. When are the tides highest ? 
A. The tides are highest when the moon and the 
sun are nearest to the earth. 

Q. 18. What causes the spring tides ? 

A. At new moon or full moon, the sun and the 
moon are in line with the earth, hence their tide- 
raising forces are combined. 

Q. 19. What causes the neap tide. 

A. At first quarter and last quarter of the moon, 
the sun and moon are at right angles with the earth, 
hence the sun's flood tide coincides with the moon's 
ebb tide so that the solar flood tide partly fills out the 
drepression of the lunar ebb tide ; likewise, the sun's 
ebb tide and the moon's flood tide then occupy the 
same position, in consequence of which, the lunar 
flood tide will be lowered by a corresponding amount. 



MARS. 41 

Q. 20. In what direction do the tides travel ? 

A. Since the apparent daily motion of the sun and 
moon is from east to west, the general direction of 
travel of the tidal wave is from east to west also. 

Q. 21. Are the tides much modified in their onward 
motion ? 

A. Yes, the tides are very much modified by the 
varying depth of the oceans and by the continents 
that stretch like gigantic barriers across the path of 
the tidal wave. 

CHAPTER XL 



Mars. $ The Satellites of Mars. 

Q 1. Where is Mars situated ? 
A. The orbit of Mars is next outside of the orbit 
of the earth. 

Q 2. How far is Mars from the sun ? 

A. The mean distance of Mars from the sun is 
141,500,000 miles; but it varies about 13,000,000 
miles each way from the mean. 

Q. 3. In what time does Mars complete a 
revolution around the sun ? 

A. Mars completes its revolution around the sun 
in 687 days. 

Q. 4. How fast does Mars move along in its 
orbit? 

A. Mars has an orbital velocity of 15 miles per 
second. 

Q. 5. How strong is the sun's light and heat at 
the distance of Mars ? 



42 MARS. 



A. At the distance of Mars the sun's light and 
heat is only about 43 per cent, as intense as it is at 
the distance of the earth. 

Q. 6. How many miles is Mars in diameter ? 
A. The diameter of Mars is 4,200 miles. 

Q. 7. What is this planet's surface and volume ? 
A. Its surface is 28 per cent, of the earth's sur- 
face and its volume is | of the earth's volume. 

Q. 8. How does the surface gravity on this planet 
compare with that of the earth ? 

A. Since Mars is less dense than the earth (0.73), its 
surface gravity comes out 0.38 of the earth's surface 
gravity; in other words, a body which weighs one 
hundred pounds on earth would weigh thirty-eight 
pounds on Mars. 

Q. 9. In what time does Mars turn on its axis? 

Mars turns on its axis once in 24 hours 37 minutes 
22.67 seconds; its day is therefore a little longer than 
a day on earth. 

Q. 10. Is the equator of Mars inclined to the plane 
of its orbit? 

A. Its equator is inclined to the plane of its orbit 
by an angle of 24° 50^; the seasons, therefore, in as 
far as being dependent on the inclination of the 
equator, must resemble the seasons here on earth. 

Q. 11. How does Mars appear when viewed with a 
telescope? 

A. Considerable detail of this planet's features has 
been observed with large telescopes. 



MARS. 48 

Q. 12. Which are some of the objects viewed? 

A. The white polar caps, patches of bluish or 
greenish shade forming about three-eighths of the 
planet's surface, large areas of orange shade, the so- 
called canals, and the oases are the principal features 
observable on Mars. 




Fig. 16. Telescopic Views of Mars. 

Q. 13. What interesting change is noticed in the 
polar caps? 

A. The polar caps grow large when the respective 
pole is turned away from the sun, and grow small or 
even disappear altogether when, half a Martian year 
later, they are turned to the sun; in this manner they 
strongly suggest the advance and retreat of snow and 
ice during the seasons here on earth. 

Q. 14. What are the bluish or greenish patches ? 
A. It has been thought that they are bodies of 
water. 

Q. 15. What are the orange portions ? 
A. It seems quite probable that the orange portions 
are dry land. 



44 MARS. 



Q. 16. What are the canals ? 

A. These were first discovered by Schiaparelli in 
1877; Lowell and others think that they are water 
courses together with the vegetation which, they sup- 
pose, grows on either side of the canals. 

Q. 17. What are the oases ? 

A. The oases are small round patches which are 
situated at the intersections of two or more canals; 
they are also supposed to be principally due to vege- 
tation. 

Q. 18. Is Mars inhabited? 

A. Though Mars receives so very much less light 
and heat from the sun than the earth, still no other 
planet approaches terrestrial conditions as nearly as 
Mars does; it has been argued that the straightness 
of the canals and their converging to certain points 
is an indication of their having been constructed 
under intelligent direction, and that for this reason, 
rational beings must live on Mars; the argument is, 
however, not conclusive and the Martians meanwhile 
remain hypothetical beings for the inhabitants of 
the earth. 

Q. 19. How does Mars appear to the naked eye ? 
A. In the sky Mars appears as a dusky red star, 
at times it outrivals Jupiter in brightness. 

Q. 20. What causes this difference in brightness ? 

A. The planet is brightest when it is nearest to us 
or in opposition ; its average distance is then 
48,600,000 miles; it is faintest when it is farthest 



MARS. 45 

away from the earth or in superior conjunction, its 
average distance is then 234,400,000 miles. 

Q. 21. Has Mars any Moons? 
A. In 1877 Professor Hall at Washington dis- 
covered two little satellites, or moons of Mars. 

Q. 22. What did he call them ? 
A. He named them Deimos and Phobos, which 
means ^'Dread'' and ^'Terror''. 

Q. 23. How far are they from Mars ? 

A. Deimos is 14,000 miles from the center of 
Mars and goes around it in 30 hours 18 minutes; 
Phobos is only 5,800 miles away and goes around Mars 
in 7 hours and 39 minutes. 

Q. 24. What peculiarity does this short month of 
Phobos produce ? 

A. It effects that Phobos rises in the west and sets 
in the east. 

Q. 25 How large are these moons ? 

A. Deimos, the smaller one, has been estimated to 
be from 5 to 10 miles in diameter, and Phobos from 7 
to 25 miles. 

Exercise. From a calendar find out if Mars comes to opposition 
during the year. This occurs at intervals of 780 days (2 yrs. 
Ifmos.). The planet's ruddy color will help you to identify it. 
Since there are a number of fixed stars that resemble it, when 
merely viewed with the eye, one must make sure that it is the planet 
If the suspected planet changes its position relative to the other 
star in the course of a few weeks, it is Mars; if not, it is a fixed 
star, and the star-maps will tell you what star it is, and to what 
constellation it belongs. 



46 THE ASTEROIDS. 



CHAPTER XII. 



The Asteroids. 

Q. 1. What are the asteroids ? 

A. The asteroids, or planetoids, are small, planet- 
like bodies which circle around the sun in orbits of 
their own and which are situated between the orbits 
of Mars and Jupiter. 

Q. 2. When was the first asteroid discovered ? 
A. Piazzi, an astronomer of Sicily, discovered the 
first asteroid on Jan. 1, 1801, and named it Ceres. 

Q. 3. What asteroids were discovered next ? 

A. In 1802, Dr. Olbers discovered Pallas; in 1804, 
Harding discovered Juno; and in 1807, Dr. Olbers dis- 
covered Vesta, which is the brightest of the 'asteroids 
and sometimes becomes visible to the naked eye. 

Q. 4. When were some more asteroids discovered ? 

A. The fifth asteroid, Astraea, was discovered in 
1845 by Henke, in 1846 none were found, in 1847 
three were discovered, and since that time every year 
has added one or more to their number. 

Q. 5. How many asteroids have been found ? 

A. Over five hundred have been found already, 
but the great majority is very small, many are prob- 
ably not more than 10 miles in diameter. 

Q. 6. What is the total number of asteroids ? 

A. For various reasons, astronomers think that 
the total number is very large— many thousands, if 
not millions. 



THE ASTEROIDS. 47 



Q. 7. How large are the diameters of the four 
brightest asteroids ? 

A. According to Barnard's measurements, which 
are however affected by a large probable error, the 
diameter of Ceres is 485, of Pallas 304, of Juno 118, 
and of Vesta 243 miles. 

Q. 8. Which asteroid is nearest to the sun ? 

A. Eros, (433), the asteroid which was discovered 
by Witt in 1898, is nearest to the sun; its mean dis- 
ance is not quite 135,480,000 miles; this is less than the 
mean distance of Mars from the sun, but, on account 
of the comparatively large eccentricity of its orbit, 
it recedes at Aphelion more than 11,000,000 miles 
further from the sun than Mars does. 

Q. 9. How near can Eros approach the earth ? 

A. It comes at times within a distance of ISi 
million miles from the earth; this is a little more than 
half the least distance within which Venus can 
approach the earth. 

Q. 10. Is Eros of any use to astronomers ? 

A. Yes, the close approaches of Eros furnish the 
most exact means known to obtain that astronomical 
quantity, called the solar parallax, upon which all 
astronomical distances depend. 

Q. 11. What is the period of Eros? 

A. Eros completes its revolution around the sun in 
643 days. 

Q. 12. Which asteriod is farthest from the sun? 
A. Thule, (279), is the remotest of the asteroids; 



48 THE ASTEROIDS. 



its mean distance is 400,000,000 miles and its period 
is 8 years 313 days. 

Q. 13. Do the orbits of the asteroids all lie in the 
same plane ? 

A. No, some orbits are greatly inclined to the 
ecliptic and their eccentricities are so various, that 
if the orbits were represented by rigid hoops, one 
would lift them all by trying to lift any one of them, 
so much are they tangled. 

Q. 14. By what method are asteroids discovered 
nowadays ? 

A. Nowadays, the photographic method is em- 
ployed in discovering asteroids; i. e., a portion of the 
sky is photographed with a specially made and 
mounted stellar camera; the stars will appear like 
dots on the plate, but any asteroids, having a proper 
motion of their own among the stars, will show this 
motion by a small trail on the plates. 

Q. 15. What is the origin of the asteroids ? 

A. The origin of the asteroids is as yet a matter 
of speculation; one view holds, that they are the 
result of a series of explosions which took place, 
from some unknown cause, in a planet which moved 
between Mars and Jupiter; the other view, which 
seems more probable, holds, that for some reason, the 
nebulous matter of which the planets are made, failed 
to be gathered in one large planet, but instead, was 
distributed to form this swarm of miniature earths. 



JUPITER. 49 



CHAPTER XIII. 



Jupiter. 1/ 

Q. 1. At what distance does Jupiter revolve around 
the sun? 

A. Jupiter revolves at a mean distance of 483,000,- 
000 miles around the sun. 

Q. 2. In what time does this planet complete a 
revolution around the sun ? 
A. It completes a revolution in 11.86 years. 

Q. 3. With that velocity does it travel in its orbit? 
A. Jupiter's orbital velocity is a little over 8 miles 
a second. 

Q. 4. What is the intensity of the sun's light and 
heat at the distance of Jupiter ? 

A. The sun's light and heat on Jupiter is only one 
twenty-seventh as intense as it is on the earth. 

Q. 5. What is the diameter of Jupiter ? 

A. The mean diameter of Jupiter is 86,500 miles; 
its equatorial diameter is 88,200, and its polar diameter 
is 83,000 miles*. 

Q. 6. How much greater is its surface and volume 
than the surface and volume of the earth ? 

A. Its surface is 119 times, and its volume 1,300 
times as large as that of the earth ? 

* Note.— The mean diameter of a planet is found by adding the 
polar diameter to twice the equatorial diameter and dividing this 
sum by three. Evidently of the three axes of symmetry, one is 
the polar diameter and the other two lie at right angles in the 
equator. 



50 



JUPITER. 



Q. 7. Is Jupiter the largest of the planets ? 

A. Yes, Jupiter is the giant among the planets; 
its volume as well as its mass is larger than that of 
all the other planets combined. 

Q. 8. How heavy would a body which weighs a 100 
pounds on earth be on Jupiter? 

A. It would weigh 264 pounds. 

Q. 9. Does Jupiter rotate on its axis? 

A. Yes, this giant planet turns once on its axis in 
about 9 hours, 55 minutes. 




Fig. 17.— Telescopic Views of Jupiter. 

Q. 10. What is peculiar about its rotation? 

A. As upon the sun, regions near the equator rotate 



JUPITER. 51 



more rapidly than portions towards the polls; this 
would indicate that Jupiter has as yet no solid 
surface, or that all the spots and markings are simply 
features of its atmosphere. 

Q. 11. How fast do the equatorial regions move on 
account of the planet's rotation? 

A. They move with a velocity of over 26,000 miles 
an hour. 

Q. 12. Is the plane of the equator inclined to that 
of the orbit? 

A. There is an angle of only 3° between these 
planes; hence, there are no seasons on Jupiter due to 
the sun. 

Q. 13. How does Jupiter appear in a telescope? 
A. Jupiter is a beautiful object for the telescope; 
even a small one reveals a great amount of detail. 

Q. 14. Which are the most conspicuous features? 
A. The most conspicuous features are the belts, 
which have a dark color. 

Q. 15. How are they situated? 

A. They are arranged parallel to the equator; 
generally there is a broad belt on each side of the 
equator and several narrow ones alongside of these; 
the number and shape of the belts varies considerably. 

Q. 16. What are these belts supposed to be? 

A. They are thought to be cloud-forms which are 
arranged in belt-like layers on account of the rapid 
rotation of the planet. 



52 JUPITER. 



Q. 17. What other features are observed? 

A. A great number of temporary black and white 
spots have been observed; in 1878 a large spot of 
reddish hue appeared; it was 30,000 miles long and 
7,000 wide, and was situated on the south side of the 
south-equatorial belt; it was a conspicuous object for 
years, but has since become very faint. 

Q. 18. Did this spot have the same period of rota- 
tion as its surroundings? 

A. No, its rotation period was about 22 seconds 
longer than its surroundings; in consequence of this, 
the spot was like an island in a river, past which the 
current drifts at a rate of 12 to 15 miles an hour. 

Q. 19. Is Jupiter self-luminous? 

A. Since the moons of Jupiter suffer total eclipse 
when they come into its shadow, it becomes certain 
that Jupiter is not self-luminous or at least but very 
faintly so, but it is universally held that the planet 
must still be very hot. 

Q. 20. How does Jupiter appear to the naked eye? 

A. Jupiter, the second brightest of the planets, 
appears nearly white in color; on the average it is five 
times brighter than Sirius, which is the brightest fixed 
star; and it is each year for several months one of the 
chief ornaments of the evening sky. 

Q. 21. How does this planet move among the stars? 

A. It slowly moves eastward among the stars for 

a little over 8 months, then comes to a halt, moves 



JUPITER'S SATELLITES. 53 

westward with reference to the stars for nearly 4 
months and then resumes its eastward journey. 

Q. 22. Do these forward and backward motions 
lie along a line? 

A. No, when the motions of the planets are plotted 
on a map they have the appearance of curious loops 
and kinks. 

Exercise.— When a planet is in opposition it is on the meridian 
(the north and south line) at midnight. It will, therefore, be east 
of the meridian during the evening hours. Look up in a calendar 
during what month of the year Jupiter comes to opposition. With 
this knowledge it will be an easy matter to find the planet, which, 
when once recognized, will always be known again. It will return to 
the same relative position with regard to the sun and the 
earth in 399 days, a little more than a year and a month. 



CHAPTER XIV. 



Jupiter's Satellites. 

Q. 1. How many satellites has Jupiter ? * 

A. Jupiter has five satellites which move around 

it from west to east at different distances and vVith 

different periods. 

*0n January 4, 1905, Perrine, of Lick Observatory, announced the 
discovery of a sixth satellite of Jupiter, and on January 6, he dis- 
covered a seventh satellite. Both satellites are quite faint. Their 
direction of motion is not yet definitely known. The sixth satellite 
is about 7,000.000 miles from Jupiter. Its orbit of revolution is 
considerably inclined to that of Jupiter. This satellite's period of 
revolution is about 250 days. It is estimated to be of the fourteenth 
magnitude, and Perrine thinks its diameter is 100 miles or less. 
The seventh satelHte is about 6,000,000 miles from Jupiter. Its 
orbit of revolution is quite eccentric and is inclined to that of 
Jupiter. It completes a revolution in about 200 days. This 
satellite is estimated to be of the sixteenth magnitude, and to have 
a diameter of about 35 miles. 



54 JUPITER'S SATELLITES. 

Q. 2. When and by whom were the four outer and 
larger ones discovered ? 

A. In 1610 the four outer sateUites were dis 
covered by Galileo; they were the first heavenly 
bodies ever discovered. 

Q. 3. How are these satellites distinguished ? 

A. The nearest of the four is designated by the 
Roman numeral L, the second by II., the third by III., 
and the fourth by IV. ; they are also known by the 
names : lo, Europa, Ganymede, and Callisto. 

Q. 4. At what distance is L from Jupiter? 

A. The distance of I. from Jupiter is 261,000 miles. 

Q. 5. What time does it require to move around its 
primary ? 

A. It completes its revolution around Jupiter in 1 
day 18 hours 27 minutes 33.5 seconds.^ 

Q. 6. What is the distance of II. ? 
A. The distance of 11. from Jupiter is 415,000 
miles. 

Q. 7. What is its period ? 

A. The period of 11. is 3 days 13 hours 13 minutes 
42 seconds. 

Q. 8. What is the distance of III. ? 
A. The distance of III. is 664,000 miles. 

Q. 9. What is its period ? 

A. The period of III. is 7 days 3 hours 42 minutes 
33 seconds. 



JUPITER'S SATELLITES. 55 

Q. 10. At what distance does IV. move around 
Jupiter? 

A. The distance of IV. from its primary is 1,167,- 
000 miles. 

Q. 11. In what time does it complete a revolution ? 
A. It completes a revolution in 16 days 16 hours 
32 minutes 11 seconds. 

Q. 12. Who discovered the fifth and innermost of 
Jupiter's satellites ? 

A. Barnard discovered the fifth satellite of 
Jupiter in 1892 with the big Lick telescope ; it is, 
perhaps, not 100 miles in diameter and only the large 
telescopes will show it. 

Q. 13. What is the fifth satellite's distance from 
Jupiter ? 

A. The distance of the fifth satellite is 112,500 
miles. 

Q. 14. What is its period? 

A. Its period is 11 hours 57 minutes 22.6 seconds. 

Q. 15. What are the diameters of Jupiter's moons? 
A. The diameter of I is 2,500 miles, of II 2,100, of 
III 3,550 ; and of IV 2,960. 

Q. 16. Do the moons of Jupiter also rotate ? 

A. From observations of certain markings on their 
discs it is probable that the third and fourth satellites 
rotate once during the time that they revolve around 
Jupiter ; in other words, their period of rotation is 
equal to their period of revolution. 



56 SATURN. 



Q. 17. In what plane do the satellites revolve ? 
A. They all revolve in planes which very nearly 
coincide with the primary's equator. 

Q. 18. What is the consequence of this ? 

A. The moons are eclipsed at each revolution, and 
likewise transit across the planet's disc; the fourth 
moon alone at times escapes eclipse. 

Q 19. What use has been made of these eclipses ? 

A. In 1675, Roemer noticed that the eclipses oc- 
curred successively later than predicted by the tables, 
when the distance between Jupiter and the earth was 
increasing, and, vice versa, the phenomena occurred 
successively earlier, when the earth and Jupiter were 
approaching each other; this is explained as due to the 
time it takes light to travel through space. 

Exercise. If you have a good opera glass at your disposal direct 
it to Jupiter. You will most likely see one or the other of Jupiter's 
moons as a tiny star near it. Observations for several evenings 
in succession will show that these tiny stars accompany Jupiter in 
its motion among the stars and that they themselves seem to 
move in a nearly straight line, now to the east and then to the 
west of their primary, each one having a fixed distance beyond 
which it cannot go. 



CHAPTER XV. 



Saturn, si 

Q. 1. What planet comes next after Jupiter ? 
A. It is the planet Saturn. 
Q. 2. How far is it away from the sun ? 
A. Its mean distance is 886,000,000 miles. 



SATURN. 57 



Q. 3. In how many years does it move once around 
the sun? 

A. This planet moves around the sun once in 29h 
years. 

Q. 4. At what rate does it travel in its orbit ? 
A. It travels at the rate of six miles per second 
around the sun. 

Q. 5. What is the intensity of the sun's light and 
heat at Saturn's distance ? 

A. At this distance the sun's light and heat are 
only g-V as intense as at the earth's distance. 

Q. 6. What is the diameter of Saturn ? 

A. The mean diameter is 73,000 miles; the 
equatorial diameter is 75,000, and the polar is 
68,000 miles. 

Q. 7. What is its surface and volume ? 
A. Saturn's surface is about 82 times, and its vol- 
ume 760 times that of the earth. 

Q. 8. Does Saturn rotate ? 

A. Yes, it rotates upon its axis in about 10 hours 
and 14 minutes. 

Q. 9. Is this planet's equatorial plane inclinea to 
the plane of its orbit ? 
A. It is inclined by an angle of about 27"". 

Q, 10. How does Saturn appear in a telescope ? 
A. Saturn is a unique and splendid object in the 



58 



SATURN. 



telescope, and the first view generally draws forth an 
exclamation of wonder and delight. 




Fig. 18— Saturn and Its Rings. 

Q. 11. What is most striking in its appearance ? 



SATURN. 59 



A. Its belts, which are however not as conspic- 
uous and changeable as those of Jupiter; and above 
all, its magnificent ring-system. (See Fig. 18) 

Q. 12. How many rings are there ? 

A. There are three concentric rings ; the two outer 
ones are bright, but the one nearest to the planet is 
dusky, and is therefore sometimes called the gauze or 
" crape '' ring. 

Q. 13. Where are the rings situated ? 

A. They are situated in the planet's equator. 

Q. 14. What are the dimensions of the rings? 

A. The outer ring has an exterior diameter of 
168,060 miles and it is a little more than 10,000 miles 
wide ; between it and the middle ring is a clear space 
about 1,600 miles in width ; the middle ring is about 
16,500 miles wide ; immediately joining the middle 
ring is the dusky ring, which is about as wide as the 
outer ring ; hence there is left a clear space between 
the inner edge of this ring and the planet's surface, 
which has a width of about 9,000 or 10,000 miles. 

Q. 15. How thick are the rings ? 

A. As compared with their other dimensions, the 
rings are very thin ; the thickness does probably not 
exceed 100 miles. 

Q. 16. Of what are the rings composed ? 

A. They are composed of a swarm of separate 
particles, the outer particles revolving more slowly 
than the inner ones. 



60 SATURN. 



Q. 17. Do the rings contain much matter ? 

A. Since the rings produce no appreciable pertuba- 
tions in the motion of the sateUites, their mass must 
be quite small. 

Q. 18. Do the rings ever become invisible ? 

A. Since the plane of the rings always remains 
parallel to itself as Saturn journeys around the sun, 
it follows that the edge of the rings is turned towards 
the earth twice at every revolution ; when the rings 
are turned edgewise towards the earth they become 
invisible for a few days, even in the largest telescopes; 
some time before and after, they appear like a thin 
needle of light, the planet then resembles a bright 
ball which is pierced by a luminous knitting-needle. 

Q. 19. At what intervals do the rings become 
invisible ? 
A. They become invisible about once every 15 years. 

Q. 20. When will they disappear next ? 
A. They will disappear again in 1907. 

Q. 21. How does Saturn appear among the stars? 
A. Saturn generally appears like a star of the first 
magnitude, and has a decided yellowish tint. 

Q. 22. How does Saturn move among the stars ? 
A. It moves eastward for about 7i months and 
westward for about 4J. 

Q. 23. Does this planet depart far from the 
ecliptic ? 



THE SATF.LIJTES OF SATURN. 61 



A. No, Saturn never departs more than about 21^^ 
from the annual path which the sun traverses in the 
sky. 

Exercise— Determine by a calendar in what constellation Saturn 
is at the time. An aid for identification is the absence ol' the 
twinklino: (unless the planet is too near the horizon) which is 
characteristic of the fixed stars. All the planets shine with a 
steady light. Mark down Saturn's position with reference to tiie 
brighter stars of the constellation. A year later you will find 
Saturn a little east of where you located it in your drawing, and 
probably it is still in the same constellation, as it takes Saturn 
about 2J years to move through one of them. Saturn is the 
remotest planet which was known to the ancients. 



CHAPTER XVI. 



The Satellites of Saturn. 

Q. 1. How many satellites has Saturn ? * 
A. Saturn has nine satellites. 

Q. 2. What are their names in the order of dis- 
tances from Saturn? 

A. Their names are: Mimas, Enceladus, Tethj^s, 
Dione, Rhea, Titan, Hyperion, Japetus, and Phoebe. 

Q. 3. What is the distance from Saturn and the 
period of revolution around Saturn of Mimas? 

A. The distance of Mimas is 117,000 miles; its 
IDeriod of revolution is 22 hours 37 minutes. 

*A tenth satellite of Saturn was discovered by Prof. W. C. 
Pickering. It has an estimated diameter of 2U0 miles, and is 
beyond telescopic vision, its period of revolution is 21 days, 
which is a little less than that of Hyperion, and the motion is direct. 
The plane of its orbit is considerably incHned to the plane of the 
rings. 



62 THE SATELLITES OF SATURN. 

Q. 4. What is the distance and period of Enceladus? 
A. The distance of Enceladus is 157,000 miles; its 
period is 1 day 8 hours 53 minutes. 

Q. 5. Who discovered these moons ? 
A. Sir William Herschell discovered these two 
in 1789. 

Q. 6. What is the distance and period of Tethys ? 
A. The distance of Tethys is 186,000 miles; its 
period is 1 day 21 hours 18 minutes. 

Q. 7. What is the distance and period of Dione ? 
A. The distance of Dione is 238,000 miles; its 
period is 2 days 17 hours 4 minutes. 

Q. 8. When were Tethys and Dione discovered ? 
A. They were discovered in 1684 by J. D. Cassini. 

Q. 9. What is the distance and period of Rhea? 
A. The distance of Rhea is 332,000 miles; its period 
is 4 days 12 hours 25 minutes. 

Q. 10. Who is the discoverer of Rhea ? 

A. Cassini discovered the satellite Rhea in 1672. 

Q. 11 What is the distance and period of Titan? 
A. The distance of Titan is 771,000 miles; its 
period is 15 days 22 hours 41 minutes. 

Q. 12. When was Titan discovered ? 

A. Titan was discovered in 1655 by Huyghens. 

Q. 13. What is the distance and period of Hyperion? 
A. The mean distance of Hyperion is 934,000 miles; 
its period is 21 days 6 hours 39 minutes. 



THE SATELLITES OF SATURN. 63 

Q. 14. Who discovered Hyperion ? 

A. G. P. Bond of Cambridge, Mass., discovered 
Hyperion in 1848; two days later, it was discovered 
independently by Lassell of Liverpool. 

Q. 15. V/hat is the distance and period of Japetus? 
A. The distance of Japetus is 2,225,000 miles; its 
period is 79 days 7 hours 54 minutes. 

Q. 16. Who discovered Japetus ? 

A. In 1671, Japetus was discovered by Cassini. 

Q. 17. In what direction do these moons revolve 
around Saturn? 

A. Their motion is direct; i. e., they move around 
Saturn from west to east, just as the primary planets 
move around the sun. 

Q. 18. What is the distance and period of Phoebe ? * 

A. The mean distance of Phoebe is 7,996,000 miles; 
its period is 546.5 days, which is just one day short of 
a year and a half. 

Q. 19. By whom was Phoebe discovered ? 

A. Phoebe was discovered by W. Pickering, at 
Arequipa, by means of photographs, which were taken 
for the purpose of discovering just some such satel- 
lite Hke Phoebe, if there were any; this was in 1898. 

*The data regarding Phoebe are taken from Vol. LIII, No. 3, 
of the Annals of Harvard College Observatory. The data are not 
final and will probably be somewhat changed by further obser- 
vations. 



64 THE SATELLITES OF SATURN. 

Q. 20. Is Phoebe a faint object? 

A. Phoebe is an extremely faint object ; its bright- 
ness is estimated at two magnitudes fainter than that 
of Hyperion, which is assumed to be of the 14.0 mag- 
nitude ; according to Young, the Yerkes telescope, 
which has an aperture of forty inches and which is 
the largest refractor in existence, '' will barely show 
stars of the seventeenth magnitude ; '' Phoebe is 
therefore just about on the limit of visibility even for 
the largest telescopes. 

Q. 21. What is the diameter of Phoebe? 

A. " From photometric considerations its diameter 
is thought to be about 200 miles." 

Q. 22. What is peculiar about Phoebe's orbit ? 

A. Its orbit is very eccentric, the eccentricity being 
0.22 ; hence the distance of this satellite from Saturn 
varies over 1,700,000 miles each way from the mean 
distance. 

Q. 23. What is peculiar about its motion? 

A. Its motion is retrograde or opposite to the 
motion of the other satellites of Saturn. 

Q. 24. How bright would Phoebe appear from 
Saturn ? 

A. It would shine like a star of magnitude 5.2 to 
6.2, depending on its distance from that body ; in 
other words, it would appear like the faint stars that 
we can just see with the naked eye. 



URANUS. THE SATELLITES OF URANUS. 65 

CHAPTER XVIL 



Uranus, i^ The Satellites of Uranus. 

Q. 1. Where is the orbit of Uranus ? 
A. The orbit of Uranus is situated between the 
orbits of Saturn and Neptune. 

Q. 2. When and by whom was Uranus discovered? 
A. Uranus was discovered by WilHam Herschel in 
1781. 

Q. 3. Had Uranus been seen before 1781 ? 

A. Yes, Uranus had been observed and its position 
recorded by Flamsteed, Bradley, Mayer and Lemon- 
nier, ir all not less than 19 times before 1781 ; but 
these astronomers failed to recognize it as a planet. 

Q. 4. What is the distance of Uranus from the 
sun? 

A. The mean distance of Uranus is very nearly 
1,800,000,000 miles. 

Q. 5. In what time does Uranus complete a revo- 
lution around the sun ? 
A. It completes a revolution in 84 years 6.5 days. 

Q. 6. How fast does Uranus travel in its orbit ? 
A. The orbital velocity of Uranus is 4.2 miles per 
second. 

Q. 7. How much light and heat does this planet 
receive from the sun ? 

A. The sun's light and heat at Uranus is only ^^^ 
as intense as it is at the earth. 



66 URANUS. THE SATELLITES OF URANUS. 

Q. 8. What is the diameter of Uranus ? 
A. The diameter is about 32,000 miles, but this 
number is still uncertain by several thousand miles. 

Q. 9. What is the surface and volume of this 
planet ? 

A. Assuming the above number for its diameter, 
the surface is about 16 times, and the volume about 
66 times greater than that of the earth. 

Q. 10. Is the period of rotation of Uranus known? 

A. Until now, it has not been possible, on account 
of the absence of distinct markings on the surface, 
to determine in what time Uranus turns on its axis. 

Q. 11. How does Uranus appear in the telescope? 
A. It has the appearance of a small disc of sea- 
green color ; at times also very faint belts are visible. 

Q. 12. Can Uranus be seen without telescopic aid ? 

A. Yes, Uranus shines like a star of the sixth 
magnitude ; sixth magnitude stars are the faintest 
stars that are visible to the naked eye. 

Q. 13. What is the apparent motion of Uranus? 

A. Uranus always remains very close to the 
ecliptic, for its orbit is inclined to it by an angle of 
only 46^; it moves eastward for 7 months and west- 
ward for 5. 

Q. 14. How many satellites has Uranus ? 
A. Uranus has four satellites; viz., Ariel, Umbriel, 
Titania and Oberon. 



URANUS. THE SATELLITES OF URANUS. 67 



Q. 15. Who discovered Ariel and Umbriel ? 
A. Ariel and Umbriel were discovered in 1851, by 
Lassell. 

Q. 16. How far is Ariel away from Uranus, and 
in what time does it move around its primary ? 

A. The distance of Ariel is 120,000 miles: it 
moves around Uranus, once in two days, 12 hours, 29 
minutes. 

Q. 17. What is the distance and period of Umbriel? 
A. The distance is 167,000 miles; its period is 4 
days, 3 hours, 28 minutes. 

Q. 18. Who discovered Titania and Oberon ? 
A. Titania and Oberon were discovered in 1787, by 
W. Herschel. 

Q. 19. What is the distance and period of Titania? 
A. The distance of Titania is 273,000 miles; its 
period is 8 days, 16 hours, 56 minutes. 

Q. 20. What is the distance and period of Oberon? 
A. The distance of Oberon is 365,000 miles; its 
period is 13 days, 11 hours, 7 minutes. 

Q. 21. How are the orbits of the satellites of 
Uranus situated ? 

A. The orbits all lie in one plane which is nearly 
prependicular to the plane of the ecliptic, making an 
angle of 82° with it; in this plane the satellites 
revolve backwards, that is from east to west. 



68 NEPTUNE. THE SATELLITE OF NEPTUNE. 



CHAPTER XVIIL 



Neptune. tJJ The Satellite of Neptune. 

Q. 1. Which is the most remote planet of the 
solar system ? 

A Neptune is the most remote planet of the solar 
system ? 

Q. 2. What led to the discovery of Neptune ? 

A. It was noticed that Uranus did not exactly 
follow the orbit which it ought to go according to 
computations, but that it seemed to be disturbed by 
the attraction of some external planet; therefore, two 
young mathematicians, Adams and Leverrier, began 
to hunt the planet, not with telescopes, but by mathe- 
matics. 

Q. 3. Did they succeed ? 

A Yes, both determined independently the posi- 
tion of the unknown planet, and Galle, of Berlin, 
found within half an hour's searching of the region 
which Leverrier had indicated the new planet, which 
was subsequently called Neptune, this was in 1846. 

Q. 4. Had Neptune been observed before ? 

A. Yes, Neptune's position had been recorded on 
several occasions; its planetary nature was however 
not recognized and the planet had been put down as a 
small fixed star. 

Q. 5. What is Neptune's distance from the sun ? 
A. The mean distance of Neptune is 2,792,000,000 
miles. 



NEPTUNE. THE SATELLITE OF NEPTUNE. 69 

Q. 6. In what time does this distant planet com- 
plete a revolution around the sun ? 

A. It completes a revolution around the sun in 164 
years, 284.7 days. 

Q. 7. At what rate does Neptune move in its orbit! 

A. It moves with a velocity of 3.4 miles per 
second. 

Q. 8. How much light and heat does the sun send 
to Neptune ? 

A. At Neptune's distance the sun's light and heat 
is only 1/900 as intense as it is at the earth. 

Q. 9. How large would the sun appear at Nep- 
tune's distance? 

A. At the distance of Neptune, the sun would ap- 
pear too small to be seen as a disc by the eye; still it 
would shine very bright; the intensity of its light would 
be very similar to that of a large electric arc-lamp 
placed at the distance of a few feet. 

Q. 10. What is the diameter of Neptune? 

A. As with the diameter of Uranus there is still 
considerable discrepancy between the values obtained 
from the measurements by various astronomers; the 
number generally accepted is about 35,000 miles. 

Q. 11. Assuming 35,000 miles for the diameter, 
what would its surface and volume be? 

A. Its surface would be about 20 times and its 
volume 85 times that of the earth. 



70 NEPTUNE. THE SATELLITE OF NEPTUNE. 

Q. 12. Is anything known of its rotation? 

A. The time of rotation is unknown, because no 
well defined markings have ever been observed on 
Neptune. 

Q. 13 How does Neptune appear in the telescope? 

A. In the telescope Neptune appears like a small 
greenish disc. 

Q. 14. Can Neptune be seen with the naked eye? 

A. No, Neptune, shining about like a star of the 
ninth magnitude, is too faint to be visible to the naked 
eye, but a good opera glass will show it. 

Q. 15. What is Neptune's apparent motion among 
the stars? 

A. Neptune's motion among the stars is direct, 
or eastward, for 6f months and retrograde, or west- 
ward, for 5^ months of the year. 

Q. 16. Has Neptune any satellites? 

A. Neptune has one satellite which was discovered 
by Lassell in 1846, a few weeks after the discovery of 
Neptune; it has not received a name. 

Q. 17. What is this satellite's distance from Nep- 
tune, and what is its period of revolution around its 
primary? 

A. Its distance is 221,500 miles, and its period is 5 
days 21 hours 3 minutes. 




Fig. 19. Swift's Comet of 1892. 



COMETS. 71 



Q. 18. What is peculiar about its motion? 

A. Its motion is retrograde in its orbit which is in- 
clined to the plane of the ecliptic by an angle of not 
quite 35^. 



CHAPTER XIX. 



Comets. 



Q. 1. What are comets? 

A. Comets are heavenly bodies which usually con* 
sist of a bright star-like nucleus surrounded by the coma 
and accompanied by a nebulous train or tail. 

Q. 2. What is the appearance of the coma? 
A. The coma has the appearance of a small hazy 
cloud. 

Q. 3. Does every comet consist of a nucleus, 
coma, and tail ? 

A. No, faint comets frequently have neither 
nucleus nor tail. 

Q. 4. Do the comets follow any law in their 
motions? 

A. Yes, all comets move in accordance with the law 
of gravitation. 

Q. 5. Can a comet's place in the sky be predicted? 

A. As soon as a sufficient number of observations 
of a comet's position has been obtained, its future 
positions can be predicted with very great accuracy. 



72 COMETS. 



Q. 6. What is the form of their orbits? 

A. The shape of a comet's orbit, except when 
modified by a planet's disturbing attraction, u always 
either a parabola, an hyperbola, or an ellipse. 

Q. 7. When comets move in parabolic or hyper- 
bolic orbits will they ever return ? 

A. Since the parabola and the hyperbola are open 
curves, comets which move in such orbits will 
probably never return. 

Q. 8. When comets move in an elliptic orbit do 
they return ? 

A. Since the ellipse is a closed curve, comets 
which have an elliptic orbit return at regular in- 
tervals. 

Q. 9. Do the orbits of the comets which move in 
ellipses approach the circular form, as do the 
orbits of the planets ? 

A. No ; the orbits of the comets are very elong- 
ated. 

Q. 10. How are the orbits of the comets placed with 
regard to the sun ? 

A. All the orbits curve around the sun and have 
the sun situated at the focus. 

Q. 11 How are the orbits situated with regard to 
the ecliptic ? 

A. Unlike the orbits of the planets which all make 
but a small angle with the ecliptic, the orbits of the 
comets are inclined to it by angles ranging from 
0° to 90^ 



COMETS. 73 



Q. 12. From what direction do the comets come 
into the solar system ? 

A. Comets have visited us from all parts of the 
heavens. 

Q. 13. With what velocity do the comets move ? 

A. Leaving out the initial velocity which the 
different comets may have, the velocity depends upon 
their distance from the sun ; the nearer they get to 
it the faster do they move ; hence those comets 
which came nearest to the sun moved at that time 
with the greatest velocity. 

Q. 14. How close do some comets get to the sun ? 

A. About a dozen comets have perihelion distances 
of less than five million miles ; the Great Comet of 
1882 even passed within 300,000 miles of the sun's 
surface. 

Q. 15. How fast did this comet move then ? 
A. At perihelion its velocity exceeded 250 miles a 
second. 

Q. 16. Do comets change much in appearance ? 

A. Yes ; in general, a comet becomes brighter as 
it draws closer to the sun ; the head, however, 
usually contracts when it approaches the sun, but the 
tail grows larger ; there are frequently also other 
unaccountable changes, so that a comet can be 
identified by its orbit only. 

Q. 17. What is peculiar about a comet's tail. 

A. The tails of the comets are turned away from 



74 COMETS. 



the sun as though this body repelled them ; hence 
when a comet moves toward the sun the tail follows, 
and when it recedes the tail precedes -the head of the 
comet. 

Q. 18. Is the cause of this repulsion known ? 

A. The explanation of this phenomenon has for a 
long time baffled astronomers ; it has now been 
experimentally demonstrated that light exerts a small 
pressure on the bodies upon which it falls ; if the 
particles be small enough the pressure exerted by 
the sun's rays will exceed the attraction of the sun 
for these particles and hence will be driven away 
from it ; changes in the electrical condition of the 
comet very likely complicate the phenomenon. 

Q. 19. Of what is the tail of a comet composed ? 

A. The tail is formed from the material which 
is ejected from the head ; since the tail dissipates in 
space, very much like smoke from a locomotive in the 
air, it is evident that a comet will be gradually 
disintegrated by repeated visits to the sun. 

Q. 20. What is the size of the heads of comets ? 
A. The heads of comets range from less than 10,000 
miles in diameter to over 1,000,000. 

Q. 21. What are the dimensions of a comet's tail ? 

A. In several comets the length of their tails has 
exceeded 100,000,000 miles ; the tails are usually 
more or less fan-shaped and measure on their outer 
extremity several million miles across. 




Fig. 20. Naked-eye View of Donati's Comet, Oct. 4, 1858. 



COMETS. 75 



Q. 22. Do the comets contain much mass ? 

A. Comets which have come near to the earth have 
had their periods changed to the extent of several 
weeks on account of the earth's attracting mass, but 
the period of the earth's revolution, i. e., the year, has 
not been changed to the extent of one second by any 
comet's attracting mass; hence the conclusion, that a 
comet's mass as compared with that of the earth is 
exceedingly small. 

Q. 23. Do comets shine by their own light ? 

A. The spectroscope shows that the great majority 
of comets have a spectrum identical with that of cer- 
tain hydrocarbon gases when brought to incandes- 
cence; the comets therefore shine principally by their 
own light, although the energy which makes the gases 
in the comets luminous is undoubtedly, for the 
greater part at least, derived from the sun. 

Q. 24. How many comets have been observed ? 

A. There are in all about 700 comets which' have 
been recorded; of these, the orbits of about 400 
have been computed. 

Q. 25. How many of these 400 orbits were found 
to be elliptical ? 

A. Seventy five orbits are distinctly elliptical; the 
others are mostly parabolic. 

Q. 26. What are the periods of the comets which 
move in elliptical orbits ? 
A. Sixty comets have periods of less than 100 



76 METEORS AND SHOOTING STARS. 

years; of these, quite a few have a period of from 
three to eight years. 

Q. 27. How are comets named ? 

Comets are generally designated by a letter of the 
alphabet, the year of the discovery, and the name of 
the discoverer, thus ''Comet a 1904 (Brooks)'' denotes 
the first comet (6 would be the second, c, the third, etc.) 
which was discovered in 1904 and that Brooks dis- 
covered it. 

Q. 28. Who was the first to recognize that certain 
comets are periodic ? 

A. Halley, a contemporary of Newton, recognized 
that the comet of 1682 was periodic and would return 
about 1758. 

Q. 29. When will this comet, known as Halley's 
Comet, reappear again ? 
A. Its next appearance will be about 1911. 



CHAPTER XX. 



Meteors and Shooting Stars. 

Q. 1. What is a meteor ? 

A. A meteor is a mass of matter which plunges 
with great velocity from outer space into the earth's 
atmosphere and flies through the air as a bright and 
fiery body ? 

Q. 2. How do meteors appear at night ? 

A, At night, they appear as balls of fire; they are 



METEORS AND SHOOTING STARS. 77 

generally accompanied by a luminous train which 
marks out their path and which frequently remains 
visible long after the meteor has disappeared. 

Q. 3. How do they appear by day ? 

A. By day the fire-ball and luminous train are not 
visible, but white cloud-like formations are seen in 
their place. 

Q. 4. What features usually accompany a meteor 
in its flight? 

A. A continuous roaring noise is produced by the 
meteor in its aerial flight, which is reinforced at in- 
tervals by the sharp reports of explosions by which 
fragments are burst off from the main body; the 
shocks from these explosions, together with the vary- 
ing resistance of the air, make the motion of the 
meteor irregular, so that it seems to dart here and 
there at random ? 

Q. 5. When a meteor has fallen to the ground, 
what is it called then ? 

A. The remains of fallen meteors have been called 
various names, such as, aerolites, uranoliths, meteoric 
stones, siderites, etc.; but they are commonly referred 
to now as meteorites. 

Q. 6. Of what are the meteorites composed ? 

A. Most of the meteorites are of a stony nature ; 
the minerals which compose them resemble certain 
terrestial minerals of volcanic origin ; a number of 
meteorites, however, are composed nearly entirely of 
iron more or less alloyed with nickel. 



78 METEORS AND SHOOTING STARS. 

Q. 7. Of what size are the meteorites ? 

A. Of the meteorites that have been seen to fall the 
largest pieces found weigh about 500 pounds ; of the 
iron masses which were not seen to fall, but which on 
account of their location, structure, composition, etc., 
are thought to be meteoric, a number weigh a few 
tons. 

Q. 8. What is a characteristic feature of meteor- 
ites ? 

A. A characteristic feature of meteorites is the 
thin black crust which covers their entire surface, 
except in such places where pieces have burst away 
just before their fall. 

Q. 9. What causes the crust ? 

A. The friction of the air on the meteor causes so 
great a heat as to make its surface incandescent and 
to fuse it. 

Q. 10. Are the meteorites hot throughout ? 
A. No, the intense heat is only superficial and 
vanishes immediately after the fall. 

Q. 11. With what velocity do meteors strike into 
the earth's atmosphere ? 

A. Meteors strike into the earth's atm 33phere with 
an initial velocity ranging from 10 to 40 miles per 
second ; by the time the meteor reaches the earth the 
speed has been reduced by the friction of the air to 
one or two miles per second. 

Q. 12. How long is a meteor's path ? 

A. The length of the path depends upon the angle 



METEORS AND SHOOTING STARS. 79 

at which the meteor meets the earth ; it ranges from 
about 50 to 500 miles. 

Q. 13. What is the origin of meteors ? 

A. Some think that the meteors have a common 
origin with the planets and asteroids ; others think 
that they are fragments which were shot off into 
space when the volcanoes of the moon and the earth 
were young and in full vigor. 

Q. 14. What is the number of the meteorites ? 

A. Since 1800 not far from 300 newly fallen meteor- 
ites have been collected ; a vast number of meteors is 
of course, never seen in their flight, nor are their 
remains found. 

Q. 15. What are shooting stars ? 
A. Shooting stars are, in appearance at least, 
meteors on a small scale. 

Q. 16. How many shooting stars fall in a day ? 

A. It has been estimated that, if all the shooting 
stars that fall over the whole earth in 24 hours could 
be observed, their number would be somewhere 
between 10 and 20 millions. 

Q. 17. At what heights do they appear and dis- 
appear ? 

A. When first seen they are at an elevation of 
about 75 miles, and they disappear when they are 
still about 50 miles above the surface of the earth. 

Q. 18. With what velocity do they traverse the 
atmosphere ? 



80 METEORS AND SHOOTING STARS. 

A. During the time of visibility their average 
velocity is about twenty-five miles per second. 

Q. 19. What is the mass of shooting stars ? 

A. The mass of the average shooting star proba- 
bly does not amount to as much as a single grain ; 
even the brightest shooting stars are thought to 
weigh less than a quarter of an ounce. 

Q. 20. What is a meteoric shower ? 

A. When a great many shooting stars fall at any 
one place within a few hours, and where all the shoot- 
ing stars seem to diverge from one point in the sky, 
the display is called a meteoric shower. 

Q. 21. What is that point in which all the paths of 
the shooting stars, if traced backward, seem to inter- 
sect, called ? 

A. The point is called the radiant of the shower ; 
in reality, it is the vanishing point of perspective for 
the shooting stars, since these all move in lines sensi- 
bly straight and parallel for short distances. 

Q. 22. What are these meteoric swarms supposed 
to be ? 

A. Meteoric swarms are thought to be the remains 
of disintegrated comets or of such comets that are 
still undergoing disintegration. 

Q. 23. What reason is there for such belief ? 

A. It was found that certain meteoric swarms move 
in orbits around the sun which are practically iden- 
tical with those of certain comets. 



METEORS AND SHOOTING STARS. 81 



Q. 24. Which are some of these swarms and 
comets ? 

A. The Perseids, or August meteors, move in the 
same path as Tuttle's comet ; the Leonids, or Novem- 
ber meteors, follow in the path of Tempel's comet of 
1866, and the Andromedes pursue the same course as 
Biela's comet. 

Q. 25. Why are meteoric showers periodic ? 

A. Since the meteoric swarms revolve in regular 
orbits around the sun, the earth can encounter them 
only then, when the swarms are at that point of their 
orbit where the earth's orbit intersects it ; this 
happens with the Lenoids roughly every thirty-three 
years, around the thirteenth of November. 

Q. 26. What are meteor-rings ? 

A. On account of the disturbing influences which 
act upon the meteor-swarms in the solar system, the 
swarms become more and more scattered and are 
finally strewn along their entire orbit; when the 
shoals of meteors have arrived at this condition, they 
are called meteor-rings. 

Q. 27. Will a meteor-ring yield a shower every 
year? 

A. Yes, since the meteors are scattered along their 
whole orbit, the earth rarely fails to pick up some 
each year when it crosses the orbit of the meteors ; 
in general the display will not be as brilliant as in 
showers which are separated by greater intervals. 

Exercise— The Perseids, or August Meteors, afford a conven- 
ient opportunity for observing a moderate meteoric shower and 



82 AURORA BOREALIS AND ZODIACAL LIGHT. 

also for finding its radiant. The Perseids move along in a very 
broad stream, so that it takes the earth several weeks in crossing 
it, and Perseid shooting stars may be seen on almost any clear 
night in which moonlight does not interfere during the last days of 
July and the first few weeks of August. The densest portion of 
the stream is encountered around August 10, wherefore these 
shooting stars have in certain places been popularly known for a 
long time as the '* Tears of St. Lawrence." The Perseids 
resemble one another. They are yellowish and move with medium 
velocity. At that time of the year, the constellation Perseus is in 
the northeastern part of the sky during the evening hours. . If 
now the paths of these yellow shooting stars be traced backwards 
they wiir all seem to radiate from a point in the constellation 
Perseus. A handy way to locate the radiant is to draw the 
observed paths of the shooting stars on a star-map or on a celes- 
tial globe. 



CHAPTER XXI. 



The Aurora Borealis and the Zodiacal Light. 

Q. 1. What is an aurora borealis? 

A. An aurora borealis, or northern lights, is a lumi- 
nous appearance taking place in the upper atmos- 
phere. 

Q. 2. Of what form is the aurora ? 

A. Its form is very changeable, sometimes it is 
simply an arch of light which spans the sky near the 
northern horizon, frequently quivering streamers of 
various tints dart fitfully out from this arch toward 
the zenith : sometimes two, more or less, concentric 
arches are visible, or the aurora appears only in 
luminous patches ; and on rare occasions the whole 
heavens are aglow with auroral light and streamers. 

Q. 3. At what time can the aurora be seen best ? 
A. It is seen best when it occurs on a moonless 
night and when there are no clouds to interfere. 



AURORA BOREALTS AND ZODIACAL LIGHT. 83 

Q. 4. Do auroras occur only during the winter ? 

A. No, they may take place at any time of the 
year; in summer, however, a display is not easily 
noticed on account of the short nights. 

Q. 5. Where are auroras most frequent ? 

A. Auroras become more frequent towards the- 
frigid zones ; within the torrid zone they are practi- 
cally unknown. 

Q. 6. What is the aurora called in the southern 
hemisphere ? 
A. It is called aurora australis, or southern lights. 

Q. 7. Of what nature is the aurora ? 

A. The aurora is most probably of an electric 
nature, and is similar to the passage of an electric 
current through a rarified gas, which is rendered 
luminous by the current. 

Q. 8. Are auroras more frequent in some years 
than in others ? 

A. Yes, the frequency of auroral displays grows to 
a maximum once in about 11.1 years, the times of 
minimum frequency being about half -ways between 
the maxima; since the frequency of sun spots varies 
in that same period, it is believed that the two are in 
some way interdependent. 

Q. 9. What frequently accompanies a brilliant 
display of the aurora ? 

A. During a grand auroral display there is often a 



84 AURORA BOREALIS AND ZODIACAL LIGHT. 

so-called magnetic storm ; while such a storm lasts, 
magnetic needles vibrate restlessly to and fro, and 
telegraph lines, especially such as run east and west, 
are traversed by powerful electric currents. 

Q. 10. What is the zodiacal light? 

A. The zodiacal light is a disc of faint light sur- 
rounding the sun and lying along the plane of the 
ecliptic. 

Q. 11. When is it best seen? 

A. It is best seen in the evening from February to 
April as a triangular hazy light with its base toward 
the sun ; the other half is best seen in the morning 
during the autumnal months. 

Q. 12. In what regions is it seen brightest ? 

A. It is seen brightest in the equatorial regions 
where it can be traced across the entire sky; there is 
a peculiar, slightly brighter diffuse spot in it which 
always keeps exactly opposite to the sun and which is 
known under the technical name of "Gegenschein" or 
"counter-glow.'' 

Q. 13. What is the nature of the zodiacal light? 

A. The most generally accepted opinion is, that it 
is the reflected sunlight from myraids of small 
meteoric bodies which circle around the sun in orbits 
more or less parallel to the plane of the ecliptic. 



THE FIXED STARS. 85 



CHAPTER XXII. 



The Fixed Stars. 

Q. 1. Which are the fixed stars ? 

A. The fixed stars are those stars which keep the 
same relative positions among themselves from year 
to year. 

Q. 2. How many of these fixed stars are visible to 
the naked eye ? 

A. There are not more than 6000 or 7000 stars 
which are bright enough to be distinguished by the 
naked eye; of these only about one third are visible 
at any one time. 

Q. 3. What is the estimated number of fixed stars 
which is visible in the largest telescopes ? 
A. The number is estimated at 100,000,000. 

Q. 4. What is the nature of the fixed stars ? 

A. The fixed stars are suns shining by their own 
light; some are larger, others are smaller, and some 
are many times brighter, while others emit less light, 
than our sun. 

Q. 5. How far are the fixed stars away from the 
solar system ? * 

A. The distance of most fixed stars is so great 
that it has been impossible to deduce a value for it; 
the nearest star known so far is Alpha Centauri, which 

* See Alphabet, Greek, in Appendix IL 



86 THE FIXED STARS. 

is separated from us by a distance of 4.4 light years ; 
i, e., the Hght sent out by that star, is on the way 4.4 
years before it reaches us. 

Q. 6. What is the equivalent of a light year in 
miles ? 

A. A light year is equal to a distance of over five 
trillion eight hundred billion miles, or about 63,000 
times the distance of the earth from the sun. 

Q. 7. Are the stars absolutely fixed in their posi- 
tions ? 

A. By no means; they are very slowly but steadily 
changing their apparent positions, so that after 
many thousand years their configurations will have 
been greatly altered. 

Q. 8. Why is this apparent shifting so slow ? 

A. It is so slow on account of the vast distances 
which separate us from the stars; in reality, the stars 
are in rapid motion. 

Q. 9. How great is their velocity ? 

A. The star called '1830 Groombridge'' has a veloc- 
ity exceeding 200 miles a second; the star called ''61 
Cygni" moves along at a speed of about 51 miles a sec- 
ond; and many other stars have been found to possess 
velocities ranging from a few miles to more than a 
hundred miles a second. 

Q. 10 In what direction do the stars move ? 
A. The stars move in all directions; it has been 
found that certain groups of stars, as the Pleiades, 



THE FIXED STARS. 87 

have a common motion; astronomers have therefore 
concluded from this and other considerations that 
such groups are physically related. 

Q. 11. Can the velocity with which a star ap- 
proaches the earth or recedes from it be measured ? 

A. Yes, by measuring the displacement of the 
lines in the spectra of the stars it has become possible 
to measure their ''motion in the line of sight'', or their 
''radial velocity.'' 

Q. 12. How are the stars classified with regard to 
their brightness ? 

A. They are classified by magnitudes; the greater 
the number of the magnitude the fainter the star; 
sixth magnitude stars are just visible to the naked 
eye. 

Q. 13. By how much do the magnitudes differ in 
brightness ? 

A. Any magnitude is about 2J times brighter 
than the next fainter magnitude. 

Q. 14. What are zero and negative magnitudes ? 

A. Some stars are brighter than those which have 
been called stars of the first magnitude; zero magni- 
tude stars are 2i times brighter than first magnitude 
stars, and stars of magnitude-1 are 2i brighter 
than zero magnitude stars. Arcturus is a star of 0.2 
magnitude, and Sirius is a star of-1.4 magnitude. 

Q. 15. How are stars designated ? 

A. Various modes of designation are in use; a 



THE CONSTELLATIONS. 



goodly number of the bright stars have proper names; 
the stars contained in a star-catalogue have each a 
number assigned them in the catalogue; in the var- 
ious constellation the stars are designated by a letter 
of the Greek alphabet, when this is exhausted, Roman 
letters, and finally numbers are used. 



CHAPTER XXIII. 



The Constellations. 

Q. 1. What is a constellation ? 
A. A constellation is a group of stars which repre- 
sents some imaginary figure. 

Q. 2. How many constellations are there ? 

A. There are about 67 constellations which are 
recognized now; 48 are of great antiquity, the re- 
mainder was introduced in modern times to take up 
space left unoccupied by the older constellations. 

Q. 3. Where are most of the new constellations 
situated ? 

A. They are near the south pole of the heavens; 
the stars in this region of the sky are not visible in 
northern latitudes and are therefore practically un- 
known to people living in these regions. 

Q. 4. What are circumpolar constellations ? 
A. They are the constellations nearest to either 
the north or south pole; to an observer in northern 



THE CONSTELLATIONS. 89 

middle latitudes the northern circumpolar stars never 
set, and a corresponding part of the south circum- 
polar sky never rises; vice versa this is true for an 
observer in southern middle latitudes. 

Q. 5. What is the Zodiac? 

A. The Zodiac is that circle of twelve constella- 
tions which lies along the ecliptic; the sun, moon, and 
planets always remain within the zodiac. 

Q. 6. Which are the constellations of the zodiac ? 

A. They are: Aries, the Ram, Taurus, the Bull, 
Gemini, the Twins, Cancer, the Crab, Leo, the Lion, 
Virgo, the Virgin, Libra, the Scales, Scorpius, the 
Scorpion, Sagittarius, the Archer, Capricornus, the 
Goat, Aquarius, the Water Carrier, Pisces, the Fishes. 

Q. 7. How are the constellations most easily and 
readily studied ? 

A. The simplest way is to compare the stars in the 
sky with a star-map; the more striking constellations 
like Orion, Taurus, Leo, Ursa Major, etc., will be rec- 
ognized with very little difficulty. 

Q. 8. How is the celestial north pole located ? 

A. The pole lies in a vertical plane which runs 
due north; in this plane the pole is elevated above 
the horizon by an angle equal to one's latitude; if, 
for instance, one lives in latitude 30° North, the pole 
is elevated one-third the distance between the horizon 
and the zenith ; if the latitude is 45° North, the pole 
is elevated at half the distance. 



90 NORTHERN CIRCUMPOLAR CONSTELLATIONS. 

Q. 9. Is the celestial south pole located in like man- 
ner ? 

A. Yes, the answer to the foregoing question ap- 
plies word for word, if for north the word south is 
used. 



CHAPTER XXIV. 



Northern Circumpolar Constellations. 

Q. 1. What star is very close to the celestial 
north-pole ? 
A. It is Polaris, the Pole-star. 

Q. 2. In what constellation is the Pole-star ? 
A. It is in the constellation of Ursa Minor, the 
Little Bear. 

Q. 3. Of what magnitude is Polaris ? 
A. Polaris is a star of 2.2 magnitude. 

Q. 4. What characteristic group of stars is in the 
constellation Ursa Minor ? 

A. It is the ''Little Dipper''; Polaris is at the end 
of the handle which is rather much bent; there are 
no first magnitude stars in this constellation. 

Q. 5. What striking group of stars is there in the 
constellation Ursa Major, the Great Bear? 

A. It is the ''Big Dipper"; the two outer stars of 
the bowl are called the "Pointers" because a line 



NORTHERN CIRCUMPOLAR CONSTELLATIONS. 



91. 



drawn through them always, though not exactly, points 
out the polar star. 

Q. 6. Where is Draco, the Dragon ? 

A. The Dragon is situated in part between the 
Great and the Little Bear, and its head extends to 
the constellation Hercules. 




Fig. 21— Northern Circumpolar Constellations. 

Q. 7. Is this constellation easily traced ? 
A. Yes, a quadrilateral of stars forms the head, and 
a sinuous line of stars marks out the body. 



92 EQUATORIAL AND ADJOINING CONSTELLATIONS. 



Q. 8. Is there any remarkable star in this constel- 
lation ? 

A. Thuban, or Alpha Draconis was the polar star 
4,000 years ago. 

Q. 9. Where is the constellation Cepheus ? 

A. It is between Draco and Ursa Minor on one 
side and Cassiopeia on the other; this constellation 
contains nothing of especial interest. 

Q. 10. How can the constellation Cassiopeia be 
found? 

A. This constellation is on the opposite side of the 
pole from the ''Dipper'' and just as far from it; the 
bright zigzag line of stars makes it an easy object for 
identification. 

Q. 11. What other circumpolar constellations are 
there? 

A. Camelopardalis, the Giraffe, lies between the 
pole, Cassiopeia and the Great Bear; Lynx is also 
partly circumpolar; there are no bright stars in these 
constellations. 



CHAPTER XXV. 



Equatorial and Adjoining Constellations. 

Q. 1. At what time of the year is Andromeda on 
the meridian at nine o'clock in the evening ? 
A. In the early part of December. 

Q. 2. How may this constellation be recognized ? 



EQUATORIAL AND ADJOINING CONSTELLATIONS. 93 

A. It may be recognized by the nearly straight 
line made by three stars of the second magnitude, 
which stretches from Pegasus to Perseus. 

Q. 3. When is Perseus on the meridian at nine 
o'clock in the evening ? 

A. Perseus is on the meridian at 9 o'clock in the 
evening during the latter part of December. 

Q. 4. By what group of stars may Perseus be 
known ? 

A. The so-called ''Segment of Perseus'' which is a 
curved line of stars running along the ''Milky Way" 
and which extends from Cassiopeia to Auriga points 
out this constellation. 




Fig. 22— Aries and Adjoining Constellations. 



94 EQUATORIAL AND ADJOINING CONSTELLATIONS. 

Q. 5. Where is Pegasus ? 

A. Pegasus adjoins Andromeda and Pisces; it 
comes to the meridian * at nine o'clock in the evening 
during the latter part, of October. 

Q. 6. What characteristic grouping of stars is 
there in Pegasus ? 

A. It is the ''Great Square of Pegasus''; this 
square together with the bright stars of Andromeda 
forms a huge figure which resembles the shape of the 
''Big Dipper" very much. 

Q. 7. What constellation is immediately south of 
Andromeda ? 

A. Pisces, the Fishes, are south of Andromeda; 
there are no bright stars in this constellation; the 
vernal equinox, i, 6., one of the two points where the 
celestial equator and the ecliptic intersect, lies in this 
constellation. 

Q. 8. Where is Aries, the Ram ? 

A. Aries is east of Pisces; a short broken line 
made by three stars, of which the upper one is the 
brightest, is easily recognized; Triangulum, the Trian- 
gle, is a small constellation of stars arranged in trian- 
gular shape and is situated between Andromeda and 
Aries. 

* The time of arrival on the meridian of a constellation is nec- 
essarily stated quite loosely; from now on, the hour of 9 o'clock in 
the evening will be assumed when the approximate time of culmina- 
tion {i e, ; the coming to the meridian) of a constellation is given. 



EQUATORIAL AND ADJOINING CONSTELLATIONS. 95 

Q. 9. What constellation is east of Aries ? 

A. Taurus, the Bull is east of Aries; the V-shaped 
Hyades and the Pleiades, frequently called the "Seven 
Sisters' V make Taurus an easy object for beginners; 
this constellation is on the meridian during Jan- 
uary. 




Fig. 23— Orion and His Dogs. 

Q. 10. What bright star is in the Hyades ? 
A. It is the fiery star Aldebaran; this star is ex- 
actly of the first magnitude. 

Q. 11. Where is the constellation Auriga, the 
Charioteer ? 

A. Auriga is north of Taurus and Gemini; it con- 



96 EQUATORIAL AND ADJOINING CONSTELLATIONS. 

tains the 0.1 magnitude star Capella which has a 
bright yellow color; a small triangle of stars near by 
are called the Hoedi, or Kids. 

Q. 12. What constellation is east of Taurus ? 

A. Gemini, the Twins, is east of Taurus; Castor and 
Pollux the two brightest stars of this constellation 
are both between the first and second magnitudes; 
Pollux comes to the meridian 11 minutes after Cas- 
tor's culmination ; the evening culminations occur in 
the early part of March. 

Q. 13. Where is Cetus, the Whale? 

A. Cetus is just south of Pisces and Aries; an ir- 
regular pentagon of stars south of Aries marks the 
head of this huge monster. 

Q. 14. When has the glittering Orion its evening 
culminations ? 

A. Orion comes to the meridian during the latter 
part of January; it adjoins Taurus on the southeast. 

Q. 15. Which are the principal configurations of 
stars in this constellation ? 

A. Four bright stars form a large quadrilateral; 
within this figure runs a diagonal line, the Belt of 
Orion, consisting of three second magnitude stars; an 
irregular line of smaller stars extends from the belt 
southward; this constellation, once learned, cannot be 
mistaken for anything else. 

Q. 16. What names have been given to the Belt of 
Orion ? 
A. In the ^'Book of Job'' it is called the ^^Bands of 



EQUATORIAL AND ADJOINING CONSTELLATIONS. 9'i 

OrioiV'; the terms ^^ Jakob's Rod" and ^^Ell and Yard" 
are also in use. 

Q. 17. What constellations are south of Orion ? 

A. Lepus, the Hare, indicated by a small quadrila- 
teral of third and fourth magnitude stars, is just south 
of Orion; still farther south is Columba, the Dove; to 
the southwest of Orion, the river Eridanus meanders 
over the celestial plains. 

Q. 18. What constellations accompany the hunter 
Orion to the east and southeast ? 

A. They are Canis Minor, the Little Dog, and Canis 
Major, the Great Dog; Procyon is a bright star of the 
0.5 magnitude in Canis Minor, whereas Sirius, the 
^^dog-star" in Canis Major is of the —1.4 magnitude; 
Sirius is the brightest fixed star in the heavens. 

Q. 19. Where is Cancer, the Crab ? 

A. Cancer is east of Gemini; it comes to the 
meridan during March; a little quadrangle (whose 
two western stars are however quite faint) with a hazy 
spot in it, the star-cluster Praesepe, mark out this 
constellation sufficiently. 

Q. 20. What constellation is east of Cancer ? 

A. It is Leo, the Lion; the evening culminations 
occur during April; this constellation is easily recog- 
nized by the ''Sickle'' and the triangle which the two 
stars Beta and Delta Leonis make with Regulus in 
the handle of the sickle. 



98 EQUATORIA.L AND ADJOINING CONSTELLATIONS. 

Q. 21. Where is the constellation Virgo, the 
Virgin ? 

A. This constellation lies east of Leo; it comes to 
the meridian in the latter part of May; Spica, a lonely 
situated bright star of the 1.1 magnitude indicates 
where the constellation may be found. 




Fig. 24— The Lion Rampant in the Skies. 

Q. 22. What constellation stretches from south of 
Cancer beyond Virgo ? 

A. It is Hydra Vviiich has its head indicated by a 
group of small stars south of Cancer; the rest of the 
constellation may be traced by means of the irregularly 



EQUATORIAL AND ADJOINING CONSTELLATIONS. 99 



curved and broken line of stars that extends from the 
head to nearly up to Scorpius. 

Q. 23. What other constellations are in this region 
of the sky ? 

A. Monoceros, the Unicorn, fills up the gap be- 
tween Canis Major and Canis Minor; Sextans, the 
Sextant, is between Leo and Hydra; adjoining Sextans 
at the southeast is Crater, the Cup, indicated by a 
pretty semicircle of six small stars; just east of Crater 
is Corvus, the Crow, which is marked out by a small 
quadrilateral of stars; south of Monoceros and east of 
Canis Major is a part of the huge southern constel- 
lation Argo Navis, the Ship Argo; adjoining this 
portion of Argo on the east is Pyxis Nautica, the 
Mariner's Campass, and east of this is Antlia Pneu- 
matica, the Air-pump; both are modern constellations 
merely designed to take up the spaces left between 
the older constellations. 

Q. 24. What constellations are north of Leo and 
Virgo ? 

A. Between Leo and Ursa Major is Leo Minor, the 
Little Lion; just south of the handle of the ^'Big 
Dipper are Canes Venatici, the Hounds; Cor Caroli, 
the brightest star of this constellation, is easily 
located; between Canes Venatici and Virgo is the con- 
stellation Coma Barenicis, the Hair of Berenice; a line 
drawn from Cor Caroli to Denebola, the bright star 
farthest east in Leo, passes through the fine star clus- 
ter of Coma Berenicis. r rtr 



100 EQUATORIAL AND ADJOINING CONSTELLATIONS. 

Q. 25. Where is the constellation Bootes ? 

A. Bootes is east of Canes a Venatici and Coma 
Berenicis, and reaches from Ursa Major down to 
Virgo; it comes to the meridian in June; Arcturus, a 
star of ruddy hue, and one of the brightest fixed stars, 
is in this constellation. 




Fig. 25— Hercules, Bootes, and Queen Berenice's Hair. 

Q. 26. What small, but very pretty, constellation 
borders upon Bootes at the east ? 

A. This pretty constellation is Corona Borealis, 
the Northern Crown, which is very readily recognized 
by a semicircle of six stars; the open part of the figure 
lies toward the northeast. 

Q. 27. What constellation is south of Corona ? 
A. It is Serpens, the Serpent ; a sinuous line of stars 



EQUATORIAL AND ADJOINING CONSTELLATIONS. 101 

leads to Ophiuchus,* the Serpent-bearer; a lozenge- 
shaped figure, made by stars in Serpens and Ophiuc- 
hus, is one of the most characteristic groups of stars 
which strike the eye on a summer evening. 




Fig. 26— Midsummer Evening Constellations. 

Q. 28. What constellations are south of the ones 
just mentioned ? 

A. They are Libra, the Scales, and Scorpius, the 
Scorpion; the latter is very easily recognized by its 
curved line of bright stars, among which blazes the 



* Ophiuchus is also called Serpentarius. 



102 EQUATORIAL AND ADJOINING CONSTELLATIONS. 

fiery Antares, the rival of Mars in color and bright- 
ness; this star is of the 1.2 magnitude. 

Q. 29. What constellation is north of Ophiuchus? 

A. It is Hercules, which extends as far north as 
Draco; Alpha of Hercules and Alpha of Ophiuchus, 
the brightest stars of their constellations, are sepa- 
rated only by a short distance and occupy a rather 
isolated position. 

Q. 30. What constellation is east of Hercules and 
south of Draco ? 

A. Lyra, the Lyre, occupies this position ; it con- 
tains the bluish- white star Vega of the 0.2 magni- 
tude, near which is a parallelogram of fainter stars; 
Lyra culminates in August. 

Q. 31. Is there anything noteworthy regarding 
Vega? 

A. Yes, on account of the precession of the equ- 
inoxes, the celestial pole describes a circle in the 
heavens around the pole of the ecliptic once in 
about 25,800 years ; 12,000 years hence the celestial 
north pole will have moved close to Vega, which will 
then be the pole-star. 

Q. 32. Where is Cygnus, the Swan ? 

A. Cygnus is due east of Lyra; it is readily 
recognized by the great cross made by its brighest 
stars; 61 Cygni, a double star, and the apex of a 
small triangle in the northeast quarter of the cross, 
is one of our nearest neighbors; the light of this star 



EQUATORIAL AND ADJOINING CONSTELLATIONS. 103 

is on the way 8.1 years before it reaches us; Cygnus 
culminates in September. 

Q. 33. What constellations are adjacent to Cygnus? 




Fig. 27— The Swan, the Eagle, and the Dolphin. 

A. Lacerta, the Lizard, fills up the space between 
Cygnus and Andromeda; Vulpecula and Anser, the 
Fox and Goose, are immediately south of Cygnus; 
south of Vulpecula lies the small Sagitta, the Arrow, 
which has on its east side Delphinus, the Dolphin; 
a diamond shaped group of stars, frequently called 
''Jobs's Coffin" is a characteristic mark of the 
Dolphin. 

Q. 34. Where is Aquila, the Eagle? 
A. Aquila is south of Sagitta; three stars in a 
straight line of which the middle star, Altair, is the 



104 EQUATORIAL AND ADJOINING CONSTELLATIONS. 

brightest, being of the 0.9 magnitude, make the con- 
stellation an easy object for identification. 

Q. 35. What other constellations are in this 
neighborhood ? 

A. Equuleus, the Colt, a small and insignificant 
constellation adjoins Pegasus on the southwestern 
corner; parts of Aquila had been appropriated for 
special constellations, but their names are meritedly 
falling into disuse. 

Q. 36. Where is Sagittarius? 

A. Sagittarius, the Archer, is the next zodiacal con- 
stellation towards the east after Scorpius, the little 
^'Milk Dipper" is its most striking group of stars, 

Q. 37. Where is Capricornus, the Goat? 

A. Capricorn is east of Sagittarius; the pretty 
naked-eye double-star in Capricorn readily catches 
the eye in the autumn evenings; this constellation 
culminates in September. 

Q. 38. What constellation takes up the space 
between Capricornus and Pisces ? 

A. It is Aquarius, the Water-bearer; a little Y of 
small stars is the principal configuration; Piscis 
Austrinus, the Southern Fish, which contains the 
bright star Fomalhaut of the 1.3 magnitude, is south 
of Aquarius. 



SOUTHERN CIRCUMPOLAR CONSTELLATIONS. 105 



CHAPTER XXVI. 



Southern Circumpolar and Neighboring 
Constellations. 

Q. 1. Is there any bright star near the celestial 
south pole ? 

A. There is no conspicuous star very near the 
south pole ; but the southern sky is rich in constel- 
lations of surpassing beauty and interest. 

Q. 2. How do the constellations appear to an 
observer in the southern hemisphere ? 

A. The constellations appear reversed to him with 
reference to the horizon, i. e,^ what a northern 
observer calls up, a southern observer calls down; 
thus, the two stars which in north latitudes form the 
base of the parallelogram of Orion, form here the 
upper side, and Sirius though, of course, still south of 
Orion is above it ; for a northern observer the north 
pole of the heavens is permanently above the 
horizon and the south pole as much below it, whilst 
the opposite is true for the southern observer. 

Q. 3. What constellation is south of Antlia and 
Pyxis mentioned in the preceding chapter ? 

A. It is the ship Argo which extends along the 
declination circle passing through it for nearly one- 
fourth the circumference of this circle. 

Q. 4. What bright star is there in Argo. 

A. Canopus is of the —0.8 magnitude and is tho 



106 SOUTHERN CIRCUMPOLAR CONSTELLATIONS. 



second brightest fixed star in the heavens ; it comes 
about 20 minutes earlier to the meridian than Sirius, 
the brightest star, but it is over 36° farther south. 




Fig. 28— The Southern Constellations. 

Q. 5. What constellation is east of Argo ? 

A. Centaurus, the Centaur, is east of Argo ; Alpha 
of Centaurus which is a binary star and one of the 
brightest stars in the heavens is, as has been 
remarked before, our nearest neighbor. 

Q. 6. What constellation is bounded by Cen- 
taurus on the east, west and north ? 

A. The beautiful constellation Crux, the 
Southern Cross, is situated here ; the dark places, 
called the coal sacks, in the Milky Way which passes 
through this constellation heighten the splendor of 



SOUTHERN CIRCUMPOLAR CONSTELLATIONS. 107 



the closely set stars in the cross ; it culminates in 
May. 

Q. 7. Is the cross only visible in the southern 
hemisphere ? 

A. Since the Cross is more than 25° away from 
the south pole of the heavens, it follows, that the 
Cross will appear above the horizon for this distance 
north of the equator; for people living near the tropic 
of Cancer, the Cross is deprived of much of its beauty 
for the reason that it appears only just a little above 
the southern horizon. 



*- . .*' •' 




I ..-' • . / 








'h Y I> R'/ 


■•s' ;- 






\ ' 


. • ^N 


■ SHIP ARGO", / • • 


■^ - "": 


"to 


V i ^f'H . ' 




/ * ^ 1 ^~~. 


'"'""■ Z ■ : 










T^iAiVt 


peacock;'- 




'a / ' i."-''— -• 


• ''-• 






"•x ~~i • : • . 




"""-•■ - \ /^ 




• WOLF:-,; 


• 




^ 
^ 



Fig. 29— Southern Circumpolar Constellations. 

Q. 8. What use do people of southern countries 
make of the cross ? 

A. The Cross stands upright when it passes 



108 SOUTHERN CIRCUMPOLAR CONSTELLATIONS, 

through the meridian, before and after passage the 
Cross is inclined ; knowing what . inclination the 
Cross has at diif erent seasons for a fixed hour, the 
inhabitants of these regions use it as a means whereby 
to tell the time. 

Q. 9. What constellation is on the opposite side of 
the pole from Crux ? 

A. Toucana, the Toucan, is on the opposite side of 
the south pole ; north of Toucana are Phoenix and 
Grus, the Crane ; between Centaurus and Grus lie a 
number of constellations, such as Circinus, the Com- 
pass, Lupus, the Wolf, Triangulum Australe, the 
Southern Triangle, Altare, the Altar, Pavo, the 
Peacock, Indus, the Indian, and others ; the order of 
succession of the above-named constellations is more 
or less from west to east ; Altare culminates in the 
latter part of July, Indus in September, and Phoenix 
and Toucana in November. 

Q. 10. What very interesting object lies on the 
southeastern confines of Toucana, but is mostly within 
the adjacent constellation Hydrus, the Water-Snake ? 

A. This interesting object is Nubecula Minor, the 
Lesser Magellanic Cloud, which is composed of a rich 
aggregation of stars, star-clusters, and nebulas; it 
comprises an area of 10 square degrees. 

Q. 11. Where is Nubecula Major, the Greater 
Magellanic Cloud ? 

A. Nubecula Major lies partly in Dorado but 
mostly in Mons Mensa, the Table-land, which is south 



TEMPORARY STARS AND VARIABLE STARS. 109 

of Dorado and extends to within 5^ of the pole; Nu- 
becula Major, which has the same composition as 
Nubecula Minor but is brighter than it, comprises 
an area of 42 square degrees and is at a distance of 
about 20"" from the pole; a plane through Orion and 
the south pole passes through Nubecula Major. 

Q. 12. How far south does Eridanus extend ? 

A. Eridanus extends from Tauiiis at the equator 
south to 32° from the pole; at its southern extremity is 
Achernar a bright star of the 0.4 magnitude. 

Q. 13. In what constellation is the south pole of 
the heavens situated ? 

A. The south pole is in the constellation Octans, 
the Octant. 

Exercise. On some clear evening when moonlight does not 
interfere, select a place which is screened from strong artificial 
illumination and w^hich affords an unobstructed view of at least a 
considerable portion of the sky. According to the season and the 
evening hour different constellations will be on the meridian. 
With the aid of the preceding chapters and the star-maps it ought 
to be an easy matter to identifiy them. This process repeated at 
different times of the year will soon bring on an acquaintance with 
with the principal constellations visible in your latitude. An 
ordinary opera glass will be of assistance, especially for such as 
have weak eyes. 

CHAPTER XXVII. 



Temporary Stars and Varible Stars. 

Q. 1. What are temporary stars ? 

A. Temporary stars are such stars that blaze up 
suddenly, some of them becoming exceedingly bright, 
and then gradually fade away. 



no TEMPORARY STARS AND VARIABLE STARS. 

Q. 2. Do temporary stars disappear entirely ? 
A. A number still persist as faint stars, but others 
have disappeared entirely from view. 

Q. 3. How many temporary stars are there ? 

A. The number of temporary stars is about 40; the 
last bright temporary star is Nova Persei which was 
discovered by Anderson, Feb. 21, 1901. 

Q. 4. What causes these stars to blaze up and then 
fade? 

A. Different explanations have been advanced, 
but no one can be considered as universally accepted; 
it has been suggested that the star suffers some kind 
of explosion; another explanation says that two 
celestial bodies got into collision; still another, and, 
perhaps, the true explanation has it, that a star either 
very faint, or already extinct has encountered a 
nebula on its path. 

Q. 5. What are variable stars ? 
A. Variable stars are such as suffer a change in 
brightness. 

Q. 6. How are variable stars classified ? 

A. Variable stars are classified as irregular, when 
they alternately become brighter and darker without 
any apparent law; as periodic, when the changes in 
brightness occur during fixed intervals; and as con- 
tinuous, when a star slowly but continuously becomes 
either brighter or darker; of this last kind perhaps 
a dozen instances are known. 



TEMPORARY STARS AND VARIABLE STARS. Ill 

Q. 7. Which is the most notable star that varies 
in brightness without regularity ? 

A. The star Eta in Argo (not visible in the United 
States) was in 1843 second in brightness only to Sirius, 
since then with irregular fluctuations it has dwindled 
down to the seventh magnitude; a number of other 
bright stars show irregularities, but the change in 
brightness is very much smaller. 

Q. 8. How are the periodically varia.ble stars sub- 
divided ? 

A. They are subdivided into long-period variables, 
short-period variables, and variables of the Algol type. 

Q. 9. Which is a typical star of the long-period 
variables ? 

A. Mira (''the Wonderful''), which is the star desig- 
nated by the Greek letter micron in the constellation 
Cetus, is a long-period variable; during most of the 
time it is too faint to be seen with the unaided eye, 
but once in about eleven months it runs up to the 
fourth or even to the second magnitude and then de- 
creases more slowly than it increased; Fabricius dis- 
covered its variability in 1596. 

Q. 10. Do many variables belong to this class? 
A. Yes; most of the variable stars belong to this 
class; a great many of these are reddish in color. 

Q. 11. How do the short period variables behave? 

A. In these the brightness is never constant for a 

longer period of time, but the star is always either 



112 TEMPORARY STARS AND VARIABLE STARS. 

getting brighter or darker; the periods of these stars 
range from less than a day to three weeks; Eta Aquilae 
and Beta Lyrae are typical stars of this class. 

Q. 12. To what are these changes thought to be 
due? 

A. It is believed that one or more companions re- 
volving around the main star cause these changes. 

Q. 13. How do the variables of the Algol type be- 
have? 

A. These remain of constant magnitude, except at 
regular intervals they become much fainter within 
the space of a few hours, and then just as rapidly re- 
cover their former brightness; Algol, Beta Persei, thus 
shines mostly as a star of the second magnitude, but 
it sinks down to the fourth magnitude once every 2 
days, 20 hours, 49 minutes; it looses and regains 
its brightness in a little over 9 hours. 

Q. 14. What causes this variation ? 

A. It was suggested more than a century ago that 
variations in brightness as exhibited by Algol might 
be caused by the regular interposition of a dark com- 
panion which revolves around its primary; the shifting 
of the lines in the spectra of these stars has shown 
this explanation to be the true one; in some cases both 
stars are bright, but so close are they together opti- 
cally, (actually they are millions of miles apart) that 
not even a large telescope will separate them; evi- 
dently, if their orbital plane lies in the line of sight^ 
they will send us less light when one is in front of the 
other than when both are in view. 



DOUBLE AND MULTIPLE STARS. 113 

Q. 15. By what method are most variable stars 
discovered nowadays ? 

A. By means of photography; the size of the image 
of a star on a photographic plate, exposed a definite 
length of time, is nearly proportional to its brightness; 
if now the same stars are photographed at different 
times, any change in brightness will be indicated by a 
corresponding size of image. 

Q. 16. What are variable star-clusters ? 

A. Examination of photographs of star-clusters 
has brought out the fact that in certain clusters a 
large number of stars is variable, e, g,, the cluster 
called Messier 3, which is in Canes Venatici, contains 
132 variables; Omega Centauri has 122; 33 variable 
stars have been detected in cluster Messier 4, which 
is a little way west of Antares in Scorpius. 

Exercise — Try to ascertain the time at which Algol is at mini- 
mum. It will be interesting to note how much fainter it is then, 
than usual. If you have leisure to watch it, you will find that in 
three and one-half hours it recovers its regular brightness. Obser- 
vation of variable stars is a field of work well suited for amateurs, 
because a great deal can be done without any instrumental equip- 
ment whatsoever, or which requires, at the highest, the use of an 
ordinary opera glass. 



CHAPTER XXVIII. 
Double and Multiple Stars. 

Q. 1. What are double stars ? 
A. Double stars are such stars which to the naked 
eye appear as single stars, but which, when viewed 



114 DOUBLE AND MULTIPLE STARS. 

with a telescope, are found to be composed of two 
stars in close proximity. 

Q. 2. What are naked-eye double stars ? 

A. Stars which can be easily seen as double by the 
unaided eye, such as Theta Tauri, Alpha Capricorni 
and Mizar with its companion Alcor, which is the 
middle star in the handle of the Big Dipper, are 
called naked-eye doubles. 






Fig. 30. Double and Multiple Stars. 

Q. 3. How are double stars classified? 
A. They are classified as optically and physically 
double. 

Q. 4. What are optically double stars? 



DOUBLE AND MULTIPLE STARS. 115 

A. Optically double stars are those which simply 
are in line with each other and where one is much 
farther away than the other; the pole-star has a small 
star of the ninth magnitude near it which is in the 
same direction as the pole-star, but is far beyond it. 

Q. 5. What are physically double stars? 

A. Physically double stars are those w^hich move 
around a common center of gravity and move in 
elliptical orbits; these double stars are usually called 
binay^ies. 

Q. 6. Are there many binary stars? 
A. The number of binary stars already discovered 
is quite large and the list is continuously increasing. 

Q. 7. What are the periods of revolution of these 
binaries? 

A. There is a great range in the length of their 
periods, extending from a small number of years to 
several hundred; Sirius, e.g., has a period of 49 yeai^, 
its mass being 3. 13 times that of the sun, and Alpha 
Centauri has a period of 81 yeai^, its mass being 
twice that of the sun. 

Q. 8. What are spectroscopic binaries? 

A. Spectroscopic binaries are stars which cannot be 
shown as double by any telescope; the spectroscope, 
however, gives conclusive evidence that the light of 
such stars comes from two distinct sources which 
alternately approach and recede from us as they 
revolve in their orbit. 



116 DOUBLE AND MULTIPLE STARS. 



Q. 9. Are there many spectroscopic binaries? 

A. About every twelfth star of the brighter ones 
so far examined has proved to be a spectroscopic 
binary. 

Q. 10. What are the periods and the orbital veloci- 
ties of the spectroscopic binaries ? 

A. The periods range from a day to several years 
and their orbital velocities reach several hundred 
miles a second ; Beta Aurigae, for instance, has a 
period of four days, a velocity of about 150 miles a 
second, a mass more than twice that of the sun, and 
the orbit in which the two components move is about 
8,000,000 miles in diameter. 

Q. 11. What are multiple stars? 

A. There is a considerable number of cases where 
three or more stars form a physically connected 
system, these are called multiple stars; besides these 
systems, there is also a goodly number of visually 
multiple stars. 

Q. 12. Are the components of binary multiple 
systems of the same mass and brightness? 

A. Sometimes the components are very similar, but 
frequently there is great diversity in size and bright- 
ness; in a number of cases a component does not emit 
any light at all. 

Q. 13. Are there any differences in color in these 
systems? 

A. Many systems present striking contrasts in 
color; it is remarkable that in the binaries the smaller 
star is always bluer than the larger. 




Fig. 31. The Great Cluster in Hercules. 



THE GALAXY, STAR-CLUSTERS AND NEBULAE. 117 



CHAPTER XXIX. 



The Galaxy, Star-Clusters And Nebulae. - 

Q. 1. What is the galaxy, or Milky Way? 

A. The galaxy is a luminous belt of varying width 
and brightness which surrounds the whole heavens; 
from Cygnus to Scorpius it divides up into two 
streams which are nearly parallel; it also contains 
blank holes and openings and is in several places 
crossed by dark bands. 

Q. 2. What causes the sheen of the galaxy? 

A. This is caused by the combined light of a vast 
multitude of faint fixed stars which constitute the 
galaxy. 

Q. 3. How are the stars distributed in the heavens? 

A. They are densely crowded in the Milky Way; on 
either side the number decreases very rapidly; a tele- 
scope showing in its field of view 122 stars in the 
galaxy shows but ten stars at a distance of 90° from 
the galaxy. 

Q. 4. What are star-clusters? 

A. Star-clusters consist of a multitude of stars 
grouped in a small space; a cluster may contain from 
a hundred to many thousand stars. 

Q. 5. Which are some of the clusters visible to the 
naked eye? 
A. The Pleiades in Taurus, the Praesepe in Cancer, 



118 THE GALAXY, STAR-CLUSTERS AND NEBULAE. 

the double group in the sword-handle of Perseus, and 
the magnificent aggregation of stars, in the southern 
constellation Toucan are clusters visible to the naked 
eye, to which they appear, the Pleiades excepted, as 
hazy spots; a small telescope, or even a good opera 
glass will, however, show them as real clusters. 

Q. 6. What are nebulae ? 

A. Nebulae are faintly shining patches in the sky 
which the spectroscope has shown to consist of matter 
in the gaseous state in various stages of condensation. 

Q. 7. Are there many nebulae ? 

A. Yes, over 10,000 have already been discovered, 
mainly by photography, but only a very few are 
visible to the naked eye. 

Q. 8. How are the nebulae distributed in the 
heavens ? 

A. Their distribution is opposite to that of the 
stars; only a few are in or near the Milky Way, but 
their number increases rapidly as the distance from 
the galaxy increases. 

Q. 9. Which are a few of the most notable 
nebulae ? 

A. The Great Nebula in Andromeda, the Great 
Nebula in Orion, the Ring Nebula in Lyra, the Dumb- 
bell Nebula in Vulpecula, the Spiral Nebula in Canes 
Venatici and the Trifid Nebula in Sagittarius are 
some of the most noted nebulae ; the first two are 
visible to the naked eye. 




Fig. 32. The Great Nebula in Andromeda. 



THE NEBULAR THEORY. 119 

Exercise. — Count the stars in the Pleiades, of which six are 
very easily seen. Thereupon look at the group with an opera 
glass and you will notice that a great many more stars have 
become visible. Fix in your mind the positions of some of the 
brighter stars which you did however not see with the naked eye 
before, and try to see them without the opera glass. It is almost 
certain that you can now easily see one or several stars more 
because vou know beforehand where and what to look for. 



CHAPTER XXX. 



The Nebular Theory. 

Q. 1. Can natural science account for the exis- 
tence of the universe ? 

A. No; natural science is absolutely unable to 
account for the existence of the universe ? 

Q. 2. Why is natural science unable to account for 
the existence of the universe ? 

A. Because natural science is based on the laws 
and the order which are observed in things already 
existing and endowed with their various properties; 
natural science, therefore, postulates matter already 
in existence, and, just as little as matter can produce 
itself, can natural science account for the existence of 
matter. 

Q. 3. How, then, can the existence of the universe 
be accounted for ? 

A. The only rational way is to hold, that an 
infinite and ever-existent Being, Almighty God, has 
created matter and has endowed it with its various 
properties. 

Q. 4. Is it within the province of natural science to 



120 THE NEBULAR THEORY. 

investigate the successive stages of development 
through which matter has passed after being called 
into existence ? 

A. Since change and development is ever going 
on according to fixed laws, and since these laws are 
held to have been in existence as long as the matter 
existed to which they apply, it follows that it is within 
the domain of science to trace things back to their 
beginnings as far as this may be possible. 

Q. 5. What theory tries to account for the present 
arrangement of the solar system. 
A. It is the Kant-Laplace Nebular Theory. 

Q. 6. What does the Nebular Theory teach ? 

A. It teaches that at one time the material con- 
centrated in the sun and its attendant bodies, was 
distributed as a nebula over a vast space^ the nebula, 
as a whole, having a resultant motion of rotation; 
under the action of its own gravitation it began to 
concentrate and different portions separated at in- 
tervals from the main mass to form the planets; from 
these abandoned portions, in the process of further 
concentration, smaller masses would again separate 
to form the satellites of the planets until, finally, the 
solar system, as we know it, was evolved. 

Q. 7. Was any heat developed while contraction 
was going on ? 

A. It is a law of nature that the temperature of a 
gaseous nebula continually rises while contracting and 
even after it has condensed so much as to be no longer 



\ 



THE NEBULAR THEORY. 121 

gaseous, it still gives off great quantities of heat; 
Helmholtz has shown that the contraction of the sun 
is the probable source of nearly all the sun's heat. 

Q. 8. Are there any reasons which make the nebu- 
lar theory probable. 

A. Yes, there are a number of reasons which 
render the nebular theory probable; such are, for in- 
stance, the signs of past igneous action on the moon, 
the present physical conditions of the planets and their 
common direction of rotation and revolution, the 
nebulosity surrounding many stars, the similarity be- 
tween the spectra of certain kinds of nebulae and 
stars, the antithetical distribution of the stars and the 
nebulae in space, etc. 

Q. 9. If the solar system was evolved according to 
the nebular theory, how long would the sun's heat 
last? 

A. It is believed that within 5 to 10 million years 
the sun would begin to cool off rapidly and cease to be 
a source of light and heat; the solar system began its 
history perhaps not more than 20 million years ago. 

Q. 10. Did the stars also evolve from nebular 
masses ? 

A. The stars, it is assumed, are the products of 
evolution; the white and bluish stars are the youngest 
and hottest, whereas the reddish stars are thought to 
be approaching extinction. 



122 APPENDIX I. 



APPENDIX I. 



A Few Simple Problems on the Celestial Globe. 

Q. 1. What is a celestial globe ? 

A. A celestial globe is a sphere on whose surface 
the constellations are represented together with the 
principal circles by means of which the positions of 
celestial objects are defined.* 

Q. 2. Where is the earth and, therefore, the ob- 
server supposed to be situated ? 

A. The earth is supposed to be situated at the 
centre, its axis coinciding with the axis of the globe 
and its equator lying in the plane of the celestial 
equator. 

Problem L 

To determine which constellations never set and 
which never rise at any one place. 

Rule. Elevate the pole (the north pole if you live north of the 
equator, the south pole if you live south of it) above the horizon 
circle by the number of degrees of your latitude; the portion around 
the elevated pole which does not go below the horizon circle, as you 
turn the globe, never sets and the corresponding portion around the 
depressed pole never rises for that particular latitude. 

Exercise. 

On the equator the latitude is zero; by the rule the poles for an 
observer there must both lie in the horizon, hence all the stars rise 
and set at the equator. Show that on the equator the sun rises and 
sets at six o'clock throughout the year. At the poles the latitude 

* It is assumed that the globe is provided with a graduated 
meridan circle in which the axis may be given any inclination, and 
with a horizontal circle to repi esent the horizon. 



APPENDIX I. 123 



is 90°, hence by the rule the axis of the globe is perpendicular to th 
horizon circle and all the stars move in circles parallel to the hori 
zon. One half of the heavens therefore never sets and the other 
half never rises for an observer at the poles. Show that at the 
poles the sun is six months above the horizon and six months 
below it. 

Problem IL 
To find the right ascension of the sun or of a star. 

Rule. Bring the sun's place in the ecliptic or the star under the 
meridan circle; then the number of hours * counting from the vernal 
equinox eastward along the equator, or equinoctial, up to the 
meridan circle is the required right ascension. 

* One hour equals 15 degrees. 
One minute equals 15 minutes of arc. 
One second equals 15 seconds of arc. 

Exercise. 

Show that the sun's right ascension is: 

hours on March 21st ] 

4 '* May 23d 

6 ** June 21st [ (For 1906) 

12 '' Sept. 23d 

18 '' Dec. 23d 

Show that the right ascension of : 

Aldebaran (Alpha Tauri) is 4 hours, 31 minutes 
Sirius (Alpha Canis Ma j oris) is 6 hours, 40 minutes 
Arcturus (Alpha Bootis) is 14 hours, 11 minutes 
Altair (Alpha Aquilae) is 19 hours, 46 minutes. 

Problem III. 
To find the declination of the sun or a star. 

Rule. Bring the sun's place in the ecliptic or the star under the 
meridian circle; then the difference in degrees measured along the 
meridian circle between the equator and the sun or star is the 
required declination; when the sun or star is north of the equator 
the declination is positive, when south of the equator, it is negative. 

Example. What is the declination of Vega (Alpha Lyrae) when the 
reading of the meridian at Vega is 80°, 42^ and where the equator 
crosses it 50° ? The declination is 80° 42^-50°= + 38° 42^ 



124 



APPENDIX I. 



Exercise. 

The sun's place is always in the ecliptic. . Show that the sun's 
greatest declination north or south of the equator is 23J° and that 
this occurs at the solstices in June and December; likewise show 
that the sun's declination is zero, i. e., the sun crosses the equator 
at the equinoxes, in March and September. 

Show that the declination of : 



Aldebaran is +16° 19^ 
Sirius '' —16° 35^ 

Arcturus '' +19° 40' 
Altair '' + 8° ST 



Problem IV. 

The right ascension and declination of a heavenly 
body being given, to find its place on the globe. 

Rule. Take the given right ascension in the equator and bring 
this point under the meridian circle, then take a distance north or 
south of the equator along the meridian circle equal to the given 
declination; the position reached is the place required. 

Example. What is the star whose right ascension is 22 hours, 
52 minutes and whose declination is — 30° Tl The star is Fomalhaut 
(Alpha Piscis Australis) in the Southern Fish. 

Exercise. 

Locate and name the stars whose right ascensions and declinations 
are as follows : 

Right Ascension. Declination. 

1 hour, 25 minutes +88° 48' 



4 ho 


urs, 30 ' 


+16° 19' 


5 ' 


' 10 


- 8° 19' 


6 ' 


41 


-16° 35' 


10 ' 


3 


+12° 26' 


14 ' 


11 


+19° 40' 


16 ' 


' 23 


-26° 13' 


19 ' 


' 46 


+ 8° 37' 



APPENDIX II. 125 



APPENDIX 11. 



Astronomical Terms. 

Aberration of light is an apparent displacement of 
a star, owing to the velocity of light combined with 
the velocity of the earth in its orbit. 

Aerolite, see Meteorite. 

Almanac, Nautical, published by U. S. Government, 
contains current astronomical phenomena and data. 

Alphabet Greek: 

Letters. Names. Letters Names. Letters N^rnes 

A, a — -Alpha, I, I Iota, P, p Rho, 

B, /3 Beta, K, ^ Kappa, 2, a s Sigma, 

r, y Gamma, A, x Lambda, T, r Tau, 

A, d Delta, M, ^ Mu, T, y Upsilon, 

E, € Epsilon, N, ^ Nu, $, Phi, 

Z, c Zeta, H, ^ Xi, X, ^ Chi, 

H, J) Eta, O, o — — Omicron, ^, >/. Psi, 

®, 6 Theta, n, n Pi, ft, o, Omega. 

Altitude is the elevation of a celestial body above 
the horizon. 

Amplitude is the angular distance along the 
horizon from the east or west point, 

A2Jexof the sun's way— A point in the heavens 
towards which the whole solar system is moving; it is 
in Hercules. 

Aphelion is the place of a planet's orbit when 
farthest from the sun. 



126 



APPENDIX 11. 



Apogee, the point in the moon's or a planet's orbit 
which is farthest from the earth. 

Apsis, plural apsides, the farthest and the nearest 
point from the attracting body in an elliptical 
orbit. The line joining these points is called the line 
of apsides. 




opposition 
Fig. SB—Aspects Of the Planets. 

Aspects of a planet, its positions with reference to 
the sun as viewed from the earth. The principal 
aspects are: Conjunction, c5, when the planet ap- 
pears close to the sun; opposition, §, when the planet 
is in opposite direction from the sui? md comes to the 



APPENDIX 11. 127 



i meridian at midnight; quadrature, n, when the planet, 
earth and sun make a right angle at the earth. (See 
Fig. 33.) 
Asteroids, See chapter XII. 
AxiSy the line around which a body rotates. 

Azimuth, of a celestial body is the angular distance 
measured along the horizon from the north or south 
points to the vertical circle which passes through the 
celestial body. 

Binaries are double stars which revolve around a 
common centre of gravity. 

Bode's Law— An empirical rule for finding the 
approximate distances of the planets from the sun. 

Celestial Latitude is the angular distance of a 
heavenly body from the ecliptic. There it nothing to 
correspond to it on the terrestial globe. 

Celestial Longitude is the distance of a heavenly 
body measured from the vernal equinox along the 
ecliptic to the circle which passes through the object 
and is perpendicular to the ecliptic. 

Clusters. See Chapter XXIX. 

Colures are the four principal meridians of the 
celestial sphere which pass from the poles, one 
through each equinox, and one through each solstice. 

Comets. See Chapter XIX. 

Conjunction. See aspects. 

Culmination, the passage of a heavenly body 
across the meridian. 



128 APPENDIX IT. 



Day Astronomical^ commences at mean noon at 
any one place and is counted up to 24 hours; thus 
Feb. 3d, 11 a. m., is Feb. 2d. 23 hrs. of the astrono- 
mical day. 

Day Sidereal is the time between two successive 
transits of the vernal equinox across the meridian. 
The sidereal day is about 4 minutes shorter than the 
ordinary day, sidereal noon therefore comes progress- 
ively earlier from day to day gaining exactly one day 
in one year. 

Declination is the angular distance of a celestial 
body north or south of the celestial equator. 

Direct Motion is motion among the stars from west 
to east. 

Disc is the visible surface of the sun, moon or 
planets. 

Eclipses. See Chapter IX. 

Ecliptic, the apparent annual path of the sun 
among the stars. 

Elements of an Orbit, the mathematical quanities 
necessary to compute the position and motion of a 
comet, planet or satellite. 

Elongation is the angular distance of a planet from 
the sun as seen from the earth. 

Equator, Celestial, the great circle in which the 
plane of the earth's equator produced cuts the celes- 
tial sphere. 

Equinoctial, the celestial equator. 



APPENDIX II. 129 



Equinoxes are the two points where the echptic cuts 
the equator; when the sun is at these points day and 
night are equal the world over; the sun is at the ver- 
nal equinox about March 20th and at the autumnal 
equinox about September 22d. 

Faculae are brilliant, white patches on the sun's 
disc; they generally surround the sunspots and are 
best seen near the sun's limb. 

Galaxy, or Milky Way. See Chapter XXIX. 

Geocentric Position, the place of a heavenly body 
with reference to the earth's center. 

Gibbous, that appearance of the moon or an inferior 
planet between full and half full phases. 

Heliocentric Position, the place of a heavenly body 
with reference. to the sun's center. 

Horizon, Visible, the skyline. 

Horizon Rational, or simply horizon, a great circle 
of the heavens whose plane is perpendicular to the 
observer's plumbline, and passes through the center 
of the earth, its poles are the zenith and the nadir. 

Hour Angle is the angle made at the pole between 
the meridian and the arc of a great circle passing 
through the pole and the sun, satellite, planet, or 
star. 

Inclination of an orbit is the angle between its 
plane and the plane of the ecliptic. 

Inferior Conjunction, when an inferior planet is 
between the earth and the sun. See Aspects. 

Inferior Planets, Venus and Mercury. 



130 APPENDIX II. 



Intercalary Day, the extra day in a leap year. 

Julian Calendar, the reckoning of time set up by 
JuHus Caesar which ordains that every fourth year 
shall be a leap year. 

Julian Day, a method of numbering days proposed 
by Joseph Scaliger in 1582; it is much used by astron- 
omers; Jan. 1, 1906, is the 2,417,212th day of the 
Julian Period. 

Julian Epoch, Jan. 1, 4,713 B. C, the date of com- 
mencement of the Julian Period. 

Julian Period, consists of 7,980, (28x19x15,) Julian 
years of 365i days each; the year 1906 is the year 
6619 of the Julian Period. 

Leap Year, all years divisible by four, except the 
century years which must be divisible by 400. 

Librations of the Moon, are apparent slight pendu- 
lous motions of the moon due to several causes; on 
account of libration considerably more than half of 
the moon's surface (59 per cent.) is visible, the re- 
maining 41 per cent, never coming into view. 

Limb, the edge of the disk of the sun, moon, or a 
planet. 

Magnitudes of stars are their different degrees of 
brightness; the brighter the star, the smaller the 
number of the magnitude. 

Mean Distance of a planet or a satellite is one half 
the major axis of its elliptical orbit. 

Meridian of a place, a great circle passing through 
the zenith, the pole, and the north and south points of 
the horizon. 



APPENDIX 11. 131 



Meteorite, a body which falls from outer space 
towards the earth. See Chapter XX. 

NadiVy the point in the celestial sphere directly 
beneath our feet, opposite to the zenith. 

Nebula. See Chapter XXIX. 

Nodes are two points where the orbit of a planet or 
of the moon intersect the plane of the ecliptic; 
ascending node, Q, is the place where the body passes 
from south to north of the ecliptic; descending node, 
23, where the body passes from north to south of the 

ecliptic. 

Occidtation is the hiding of a star, satellite, or planet 
by the interposition of the moon or some other planet. 

Opposition. See Aspects. 

Orbit, the path of a planet, comet, or meteor around 
the sun, or of a satellite around a primary. 

Parallax is the difference of direction of a heavenly 
body as seen from two points, as the centre of the 
earth and some points of its surface, or from two 
different points in the earth's orbit. 

Penumbra, the partial shadow between the umbra, 
or region of total eclipse, and the exterior region of 
no eclipse; the half dark parts of a sunspot. 

Perigee, the point in the moon's or a planet's orbit 
which is nearest to the earth. 

Perihelion is the place of a planet's orbit when 
nearest to the sun. 

Period, the time of revolution of a heavenly body 
about its primary. 



132 APPENDIX 11. 



Periodic, recurring or returning at regular in- 
tervals. 

Perturbation is the effect of the attractions of the 
planets or other bodies upon each other which disturb 
their regular motion and distort their orbits. 

Phases, the various appearances of the illuminated 
portion of the moon or of the interior planets. 

Planets, the large bodies that revolve around the 
sun in elliptical orbits. 

Planetoids are asteroids; see Chapter XIL 

Poles Celestial, the two points on the celestial 
sphere, where the earth's axis of rotation produced 
pierces the celestial sphere. 

Precession of the equinoxes, a slow shifting from 
east to west of the equinoxes; it amounts to a little 
more than 50^^ a year. 

Quadrature, see Aspects. 

Radiant, is that point in the heavens from which 
the shooting stars belonging to a meteoric shower 
seem to diverge. It is merely a point of perspective, 
just as the rails of a straight piece of track seem to 
converge in the distance. 

Refraction is the bending of a ray of light as it 
passes through media of different densities. Re- 
fraction affecting the apparent place of a heavenly 
body is greatest near the horizon, making e. g., the 
sun appear just above the horizon when it is still below 
it; refraction decreases rapidly as the angle of eleva- 
tion increases and is zero at the zenith. 



APPENDIX 11. 133 

Retrograde Motion is an apparent motion of planets 
and satellites from east to west. 

Revolution^ the motion in a closed curve of the 
heavenly bodies around their centres of attracting 
force; a complete circuit of such a motion. 

Right Ascension is the angular distance measured 
eastward along the equator from the vernal equinox 
to the arc of a great circle which passes through the 
poles and the heavenly body; it is generally expressed 
in units of time. See Problem II, Appendix I. 

Rotation is the motion of a body around its axis. 

Satellites are moons revolving around the planets. 

Sidereal Year is the time required by the sun to 
move once around the ecliptic from any fixed star to 
the same star again. The sidereal year is longer than 
the tropical year by about 20 minutes. 

Signs of the Zodiac^ are the twelve equal parts of 
30° each into which the zodiac is divided, they are: 
Aries t, Taurus «, Gemini n, Cancer 25, Leo ^, 
Virgo trj^. Libra :^, Scorpio HL, Sagittarius /, 
Capricornus VJ, Aquarius ;r, Pisces X- Formerly the 
signs and the constellations coincided; on account of 
the precession of the equinox, however the sign of 
Aries has backed into the constellation Pisces, the 
sign of Taurus into the constellation Aries, and in like 
manner the remaining signs. 

Solstices are the two points in the ecliptic which are 
most distant from the equator. The sun passes the 



134 APPENDIX II. 



summer solstice about June 21st and the winter 
solstice about December 21st, giving the sun the 
longest and the shortest path above the horizon, 
respectively. 

Superior Planets are the planets which are farther 
from the sun than the earth. 

Synodic Period of a planet is the time required by 
a planet to pass from opposition to opposition or from 
inferior conjunction to inferior conjunction again. 

Syzygies are points of new and full moon of the 
moon's orbit; they also denote the points of a planet's 
opposition or conjunction. 

Terminator^ the dividing line between the illumi- 
nated and the dark part of the moon's or a planet's 
surface. 

Transit, the passage of a celestial body across the 
meridian; also the passage of Venus or Mercury 
across the sun's disc or of a satellite across the disc 
of its primary. 

Tropical Year is the time required by the sun to 
pass from the vernal equinox to the vernal equinox 
again. 

Umbray that part in an eclipse where the direct 
light is entirely cut off; the darkest part in a sunspot. 

Vertical Circles pass through the zenith and are 
perpendicular to the horizon; the prime vertical is 



APPENDIX II. 



the circle passing through the zenith and the east and 
west points. 

Zenith is the point overhead in the celestial sphere. 

Zodiac is a belt 18"^ wide encircling the heavens, 
the ecliptic lying along the middle of this belt; the 
planets always remain within this belt. 

Zodiacal Lights see Chapter XXL 



AUG 30!9i 



,;ig^ OF CONGRESS 




